Fluiddynamik und Turbulenz (B1) - Turbulentes Mischen

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Turbulentes Mischen bei sehr hohen Reynoldszahlen in der Bosporus Meeresstraße

Projektleitung:Dipl.-Ing. Hermann Lienhart
Beteiligte:Dipl.-Ing. Hermann Lienhart,
Förderer:
BMBF
TUBITAK
Stichwörter:Turbulence, oceanography, stratified flow, mixing, shear layer, mixing layer, two-point correlation, anisotropy of turbulence, acoustic Doppler velocimetry, modeling
Laufzeit:1.6.2011 - 31.5.2014
Inhalt und Ziele:A well-defined natural turbulent shear layer flow exists between the counter-flowing currents in the two straits connecting the Marmara Sea to adjacent seas, namely the Bosphorus and Dardanelles (these domains defining the limits of the Turkish Straits System, TSS), due to the density difference between the Aegean and Black Seas. For example in the Bosphorus Strait the heavier, more salty Aegean water flows in the lower layer towards Black Sea and the lighter Black Sea water flows in the upper layer towards the Marmara Sea. These two currents generate a shear layer with almost constant velocity gradient at the middle depth of the strait with a thickness of about 10 m or larger. Within this shear layer, turbulent mixing of scalar quantities like salt and heat takes place. The Reynolds number of turbulence is expected to attain very high values (Re lambda=3000). As a result of very high Reynolds number, Peclet number for temperature and salinity fields are also very high in the mixing layer. The constant velocity gradient, the high Reynolds number state of the flow and the mixing of two scalars (temperature and salinity) make the Bosphorus strait a unique natural laboratory for turbulence studies and, specifically, turbulent scalar mixing. In the present project, two teams from the collaborating institutes are aiming to studying turbulent scalar mixing with highly resolved measurements of turbulent velocity fluctuations and scalar quantities like salinity and temperature in the shear and mixing layer of Bosphorus strait. The measurements will serve to understand scalar mixing at high turbulent Reynolds numbers and help to validate mixing models in seas and oceans. For those purposes, the two teams are going to design and construct a dedicated measurement module. The measurement module will not only allow conducting 3- component two-point velocity correlation measurements to characterize turbulent fluxes in the shear layer but also resolve turbulent fluctuations of salinity and temperature. The developed module will deliver detailed turbulence information such as the energy, spectrum, scale distribution and anisotropy of turbulence which are resolved down to Taylor’s micro scale. This analysis would help to understand anisotropy development and the deviation of turbulence from the theoretical turbulence concept of Kolmogorov at high Reynolds numbers. The experimental data will later adopted in the modelling of turbulent mixing in the strait systems and, in general turbulence.
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1. Introduction / Background

Turbulent flow can be encountered in nature (galaxy, atmosphere, oceans, human body, etc.) and in many technological applications (combustion chambers, nuclear reactors, pipe lines, mixers, etc.). Mixing rate of flowing matter, speed of chemical reactions, level of generated sound, aerodynamic and hydrodynamic drag on transportation systems or even on buildings are all influenced by the turbulence. Hence, it is important to understand its influence on the aimed technology or on the environment. However, owing to its stochastic nature, turbulence and its influence on scalar transport can be only statistically described. Direct numerical simulations which principally can resolve all details of turbulence are limited to a narrow range influential  parameters, especially to low Reynolds numbers . Hence, there is still a strong need for turbulence research aiming at constructing better models of turbulence, so that prediction of turbulence and its influence in environmental and technological applications become accessible and more accurate.

When turbulent flows are  treated in the frame of Reynolds averaged Navier Stokes equations, one can observe that the interaction between mean velocity field with turbulent flow structures, i.e. turbulent fluctuations, has a linear nature and the interaction between turbulent flow structures is non-linear. The spatial gradient of mean velocity plays a dominant role in the turbulence production, in the redistribution of kinetic energy between the velocity components and, consequently, in scalar mixing. Its influence is best studied when turbulence is exposed to certain types of velocity gradients, like sheared and strained flows. Besides the influence of mean velocity gradients on turbulence, the anisotropy of turbulence accounts for 60 − 70% of all basic mechanisms involved in turbulence dynamics. These mechanisms can be isolated and studied most efficiently in homogeneous turbulence in which turbulence is exposed to constant mean velocity gradients, namely in sheared and strained flows.

The Turkish Straits System (TSS), consisting of the Bosphorus and Dardanelles Straits and the Sea of Marmara, provides the only mechanism of communication between the Black and the Mediterranean Seas. Counter flowing waters of the Black Sea and Mediterranean Sea are mixed by turbulent entrainment processes along their course through the Turkish Straits. In the Bosphorus Strait, the entrainment into the upper layer from below is abruptly increased when the flow is accelerated in the narrower southern reach, where the flow passes through a contraction. In contrast, the lower layer salinity decreases towards the north first by gradual entrainment within the Strait and later at an increased rate in the wide continental shelf region upon exit into the Black Sea (Özsoy et al., 2001).

The TSS is located in a region with demonstrated sensitivity to climatic changes and contrasts (Özsoy, 1999), and it is also capable of driving environmental changes in the adjacent basins disproportionate to its relative size. Among the two Straits, the Bosphorus plays a predominant role, determining local transport (Özsoy et al., 1995) and exchange (e.g. Ünlüata et al., 1990).

Figure 1 (a)  ERS-1 SAR image of the Bosphorus Strait and the adjoining Marmara and Black Sea regions (Özsoy et al., 2001), (b) The salinity and velocity profiles along the Bosphorus strait (Gregg and Özsoy, 2002) .

The salinity and the velocity profiles shown in Figure 1(b) and (c) clearly show the mixing layer of salinity and momentum. A close look to the velocity profile reveals that especially at the contraction (see Figure 1a) the velocity attains almost a constant gradient along a depth of about 15 m. Hence, locally one has a constant shear flow. Moreover, turbulence is expected to attain a Reynolds number of 3000, which is based on the Taylor’s micro scale of turbulence  and the r.m.s. of the turbulent velocity fluctuation . Owing to the high , the largest length scale of turbulence can be around 5 to 10 m, whereas the smallest scale (Kolmogorov scale) might be as small as 100 µm.  According to Mydlarski & Warhaft (1996), , the  flow   in the Bosphorus strait has a “strong turbulent” nature, since . In other words, non-linear interactions of turbulent eddies with each other are pronounced. Owing to the very high  and the two scalar gradients (salinity and temperature) the Pecklet numbers for salinity and temperature attain also very high values.

The constant velocity gradient (constant shear), the high  and the gradients of scalar variables make the two-layer flow in the Bosphorus strait a unique natural turbulence research laboratory. The investigations of the turbulence and scalar mixing due to turbulence conducted there can be used in two fold:

  • Discovering the physics of turbulent mixing process at high . Interesting issues are the turbulent flow structures and their statistics in terms of their spectra, correlation functions, structure functions, anisotropy of Reynolds stresses and the length scales.
  • Guiding the turbulence and scalar mixing modelling in the straits and, in general, turbulent mixing layers at high . Ultimately, it is expected to have accurate transport models of matter, momentum and heat which would then deliver for example fast predictions of pollution transport in TSS.

In this project, we are aiming to conduct experimental and theoretical investigations on the turbulence mixing in Bosphorus strait in detail and provide data serving to the above preeminent targets.

After the last accident oil spill accident at Gulf of Mexico (20.04.2010), it became more clear how serious can be the impact of contaminants on the environment and civilization. The estimated spill is about 5 million barrels of oil (795 million l). After the recovery of leakage (02.08.2010), it became an intriguing question where the spilled oil is located and its environmental impact. Only recently, a new study published in the journal Science (Camilli, et al. 2010) confirmed the existence of a huge plume of dispersed oil deep in the Gulf of Mexico and suggested that it has not broken down rapidly, raising the possibility that it might pose a threat to wildlife for months or even years.

The TSS is one of the busiest areas of shipping, industry, etc., as transit route between two main basins of Europe, and a productive ecosystem located in a gradient zone, and a migration pathway of marine species that is under severe environmental threat. The proposed turbulence and turbulent mixing investigations in this research are of key importance and are therefore expected to positively contribute to a series of earlier attempts to develop predictive models of the Turkish Straits System, a complex of separate models of the individual straits and the Marmara Sea basin. The details of turbulent transfers at the interface can lead to better parameterization of interfacial mixing and friction, which would not only contribute to forecasting in the complicated TSS region, but also to the presently unresolved problem of Mediterranean – Black Seas coupling, so as to improve the quality of forecasts in the adjacent seas.

The modelling of turbulent mixing in Bosphorus strait is also intended to be used mainly for numerical predictions of contaminant transport and to assess the environmental impact on the TSS, which has a very busy traffic, including tankers carrying vast amounts of oil and all sorts of chemical materials. At the moment, IMS-Erdemli leads an “Excellence Network for Meteorology and Oceanography (MOMA)” in Turkey. Numerical tools are developed to predict contaminant transport with this initiative. A better understanding of the TSS could also lead to more accurate estimations of the TSS influence on the adjacent basins. It is through the achievement of this objective, that the issue of inter-basin coupling will be better understood, and therefore better predicted. Improved representation of the Straits response can truly contribute to the operational forecasts of the Mediterranean and Black Seas. The outputs, in the form of forecasts of two-way fluxes (upper and lower layer) into the adjacent seas, and total transports between layers in the straits and between different compartments of the TSS, in addition to prediction of sea-level differences between the Mediterranean and Black Seas, will improve forecasts in the adjacent areas and help to establish the basic elements of the hydrological cycle affecting both basins.

2. Scientific objectives and Scope

The subject of the proposed investigations is the turbulent mixing in the Bosphorus strait. The investigators have two main objectives in mind:

O1. Understand the turbulent mixing phenomena in the stratified two-layer shear flow in the Bosphorus strait by experimental means

O2. Generalize the observations

  • for modelling the transport phenomena in the straits
  • for turbulence modelling in the framework of homogeneous turbulence.

The following questions is planned to be answered in the context:

Q1. What are the length and time  scales turbulent velocity, salinity and temperature in the shear layer (mixing layer)?

Q2. How do the turbulence statistics (Reynolds stresses, velocity and scalar spectra, etc.)   behave at high  compared to the known theoretical and experimental results

Q3. What is the anisotropy of turbulence among the all scales of turbulence in the shear layer?

Q4. How can the data obtained in this project utilized in the models of strait flows and turbulence?

Figure 2 Utilization processes of the results stemming from the proposed type of research



3. Innovation and originality of the approach

One important focus of LSTM-Erlangen is turbulent flows, their modelling and application. IMS is a unique institution in Turkey working on physical oceanography whose main focus is the seas surrounding Turkey. Owing to presence of two layered constant shear flow and high ,  the Bosphorus Strait can be accepted to be a unique natural laboratory for oceanographers and turbulence researchers. Thank to complementary capabilities and common interest on turbulence research of both applicant institutes, it can be possible for the first time to conduct systematic investigations of turbulence mixing in the Bosphorus strait. Moreover, the data would serve not only to understand turbulent mixing in the Bosphorus strait but also to answer certain important aspects of turbulence and, consequently, improve turbulence models in a more general frame.

4. Relevance for EU integration process

This collaborative project will be the first of his kind. It will serve as a basis for larger scale cooperation. Hence, the institutes can collaborate later in environmental and physical sciences. In a broader sense, the cooperation can cover basic and applied research in:

  • environmental sciences (meteorology, oceanography),
  • generation and transport of energy
  • marine & aerospace technology

which are the prior focus of the current European 7th frame programme. Hence, both institutes would have another chance to fuse their efforts in the European research and development community.

5. Approach

The study of turbulent mixing in the Bosphorus strait requires, first, the measurements of turbulent fluctuations of velocity, salinity and temperature and, second,  the adoption of the data obtained in model development and improvement. Hence, the aims of the proposed project can be achieved in three main steps, which are of experimental and theoretical character. In the first step, the measurement module will be designed, constructed and tested. In the second part, measurements will be conducted and in the third step modelling of the strait flows and turbulence in general frame will be done.

The measurements of all fluctuating variables, namely salinity, temperature and velocity,  will have a high time-resolution so that turbulence scales down to Taylor’s micro scale can be resolved. Two-point velocity correlation measurements will be conducted for all the variables. These kind of measurements are needed to resolve the size of the scales in the direction of separation (see figure below). Through the usage of auto-correlation functions and Taylor’s frozen turbulence assumption, the scales in the flow direction can be evaluated (e.g. Ertunç, 2007a, 2008). For the two-point velocity correlation measurements, two acoustic Doppler velocimetry systems (ADV), which can resolve 3 components of the velocity, are planned to be used as sketched in Figure 3. Two fast conductivity and temperature sensors (not shown in Figure 3) are going to be mounted on the ADV systems, so that simultaneous measurements of all variables can be conducted.

In order to monitor the deviation of the turbulent fluctuations from Gaussian distribution, it is common to analyze the probability density function of the structure functions of any variable. The structure function of any fluctuating variable. Therefore, the simultaneous measurements of variables at two separate points will allow to evaluate structure functions.

As the measurement module will be exposed to highly turbulent shear flow, it is expected that the module itself will be non-stationary. In order to correct the velocity measurements for the unpredictable movements of the ADV’s, two 3-component acceleration sensors will be developed and mounted on the lower and upper ADV platforms. The data acquisition from the acceleration sensors will be achieved by an underwater acquisition system available at IMS (not shown in the sketch).

Proposed two-point correlation turbulence measurement system

Figure 3 Suggested measurement module equipped with two ADV, which are connected to two platforms separated with a distance of d. The distance d can be adjusted, so that two pint correlation measurements can be conducted.

 

From the data obtained, one can evaluate the functions listed below:

 

Spectrum

Two-point correlations (Structure fuctions)

Auto correlation

Length scales

Velocity (3C)

Longitudinal & transverse

Temperature

Longitudinal

Salinity

Longitudinal

The data later will be used in the models of  strait flows and to check the validity of the numerical predictions in the straits. The simulation tools needed for the predictions are already available at IMS-Erdemli and being continuously developed in the context of MOMA project. In a more general frame work given by Banerjee et al. (2009a, 2009b), the data will be used to improve a turbulence model.

 

6. Present state

In July 2011, the kick-off meeting was made at IMS METU in Erdemli Turkey. Basic discussion points were

  • The measurement system
  • Adaptation of the measurement system to Bilim-2
  • Workshop organization
  • Exchange of scientists

 

Research vessel of IMS METU Team discussion in Bilim-2
Figure 4 (a) The project team in front of the research vessel (Bilim-2) of IMS METU

(b) Team is dicussing the possible meassurement locations in Bosphorus strait

By the end of November 2012, the design of the electronic components of the measurement system is finalized and most of the componenets were purchased. The measuremnt system is composed of two measurement units, one of which is the main unit and the other s called twin. One pairs of sensors are attached to them, such that eaxch system has one of them. Accordingly, the complete system is composed of 2 acoustic Doppler annemometers (Nortek), 2 fast concentration sensors (Seabird), 2 fast temperature sensors (Seabird), 2 depth sensors(Seabird) and 2 orientation sensor(VectorNav). The data acquisition of analog and digital signals over RS232 and RS422 ports is made by a NI-CompactRio system. In order to have simultanaous acquisition of all signals all sensors will be triggered with 200 Hz, tough some of the sensors can not reach to that speed.The power and data are transmitted by armoured towing cables. For the data transfer, single mode fiber cable is going to be employed in half duplex mode. Two units will be connected by a  CAT5E cable. All cables and connecteors are selected accordoing to the underwater application standards.  At present, design of the structure is being made at LSTM-Erlangen.

Schematics of the electronic components of the measurement system
Figure 5 Schematics of the electronic componenets of the measurement system