Flow Control: The Future

The ability to actively or passively manipulate a flow field to effect a desired change is of immense technological importance. In this article, methods of control to achieve transition delay, separation postponement, lift enhancement, drag reduction, turbulence augmentation, or noise suppression are considered. The treatment is tutorial at times, making the material accessible to the advanced graduate student in the field of fluid mechanics. Emphasis is placed on external boundary-layer flows although applicability of some of the methods reviewed for internal flows will be mentioned. An attempt is made to present a unified view of the means by which different methods of control achieve a variety of end results. Performance penalties associated with a particular method such as cost, complexity, or trade-off will be elaborated.

The subject of flow control is broadly introduced in this first chapter, leaving much of the details to the subsequent chapters of the book. The ability to actively or passively manipulate a flow field to effect a desired change is of immense technological importance, and this undoubtedly accounts for the fact that the subject is more hotly pursued by scientists and engineers than any other topic in fluid mechanics. In this chapter classical tools of flow control are emphasized, leaving the more modern strategies to the following chapter. Methods of control to achieve transition delay, separation postponement, lift enhancement, drag reduction, turbulence augmentation, or noise suppression are considered. The treatment is tutorial at times, making the material accessible to the advanced graduate student in the field of fluid mechanics. Emphasis is placed on external boundary-layer flows although applicability of some of the methods reviewed for internal flows will be mentioned. An attempt is made to present a unified view of the means by which different methods of control achieve a variety of end results. Performance penalties associated with a particular method such as cost, complexity, or trade-off will be elaborated.

The ability to actively or passively manipulate a flow field to effect a desired change is of immense technological importance. In this article, methods of control to achieve transition delay, separation postponement, lift enhancement, drag reduction, turbulence augmentation, or noise suppression are considered. Emphasis is placed on external boundary-layer flows although applicability of some of the methods reviewed for internal flows will be mentioned. Attempts will be made to present a unified view of the different methods of control to achieve a variety of end results. Performance penalties associated with a particular method such as cost, complexity, or trade-off will be elaborated.

The excellent drag reduction effect of the bubble drag reduction technique has been proved through many experiments since it was proposed. In this paper, the authors investigate the bubble-turbulence interaction and the corresponding drag reduction effect with a two-way coupled Euler–Lagrange code. The liquid phase is simulated by using a large eddy simulation method with the immersed bubbles treated using a nonlinear collision model to accurately simulate the bubble–wall interaction. A Gaussian distributed method is adopted to obtain the void fraction and interphase forces in the two-way coupled algorithm. Two typical wall-bounded turbulent flow problems (turbulent channel flow and boundary layer flow) are simulated to validate the accuracy and stability in bubbly flows and investigate the drag reduction mechanism. First, the effect of bubbles on the turbulent flow is studied in the channel flow cases in which the bubbles are observed attaching to the upper plate and swaying in the spanwise direction. In this case, Reynolds stress near the wall is decreased, which contributes to the drag reduction. Moreover, drag reduction of a turbulent boundary layer flow with bubble injection is studied in which the drag reduction under different air flow rates is in good agreement with experimental results. The contribution of turbulence and different liquid forces to the migration of bubbles away from the wall is investigated. The bubble trajectory in the turbulent boundary layer is divided into three distinct stages and discussed in detail finally.

There is a need for a sensor to measure global, spatially and temporally resolved wall shear stress on wall bounded ∞ows in various engineering flelds. A wall shear stress sensor using a micro pillar array made out of silicone rubber is presented. This sensor is based on the principle that, if such a pillar is inside the viscous sub layer the de∞ection of the pillar is proportional to the drag forced experienced by the pillar, which in turn is proportional to the wall shear stress. The displacements of individual pillars in the array are tracked to obtain the wall shear stress fleld in a turbulent boundary layer ∞ow. Design and manufacturing considerations are discussed along with typical sensor calibrations in a fully developed turbulent channel ∞ow. Based on the resolution needed the sensor can be tuned for various applications. To demonstrate the feasibility of these types of sensors, the turbulent statistics in a fully developed channel ∞ow is studied. The instantaneous wall shear stress distribution around a cylinder in cross ∞ow was also mapped. I. Introduction The measurement of skin friction or wall shear stress is important for several everyday engineering problems. The time averaged values of the wall shear stress are a measure of the global state of wall bounded ∞ows and is used to determine quantities such as skin friction drag on a body moving in a ∞uid. The time resolved measurement of wall shear stress gives an estimate of the turbulent activity in the ∞ow and describes the momentum transfer events between the body and the ∞uid. The instantaneous wall shear stress is a foot print of the individual unsteady ∞ow structures that transfer momentum to the wall. 14 Wall shear stress is signiflcant especially in improving the performance and e‐ciency of transportation vehicles by reducing drag. In the airline industry skin friction drag accounts for about 45 % of the drag on an aircraft at cruise conditions. 4 Measurement of skin friction, thus assumes signiflcance as a reduction in drag directly results in a reduction in fuel consumption. Likewise, skin friction is responsible for a great part of the power expended in pumping oil and natural gases through pipes across countries and even continents. These ∞ows fall under the broad classiflcation of ∞ows called high Reynolds number ∞ows. The flnancial implications of measuring wall shear stress in a spatially and temporally resolved manner in a high Reynolds number ∞ow is hence signiflcant. Skin friction is also an important measured quantity because it helps in characterizing the state of the turbulent boundary layer, which is important both to the fundamental understanding of these ∞ows and also to assist in the fleld of ∞ow control. Flow control deals with the controlling of these ∞ows by using spatially distributed values of the instantaneous wall shear stress in manner such as to efiect changes in the boundary

The present study addresses the drag reduction due to the repetitive laser induced energy deposition over a flat-nosed cylinder. Irradiated laser pulses are focused by a convex lens installed in side of the in-draft wind tunnel of Mach 1.94. The maximum frequency and power of the energy deposition is limited up to 50 kHz and 400 W. Time-averaged drag force is measured using a low friction piston which was backed by a load cell in a cavity as a controlled pressure. Stagnation pressure history, which is measured at the nose of the model, is synchronized with corresponding sequential Schlieren images. Amount of drag reduction is linearly increased with input laser power. The power gain only depends upon the pulse energy. A drag reduction about 21% which corresponds to power gain of energy deposition of approximately 10 was obtained. n the past decade, advanced technology using energy deposition, which is produced by laser beam, micro wave or electric spark has been suggested actively to attempt the flow control. The energy deposition technique is applied mainly in fluid engineering filed such as a modification of shock structure, active control of boundary layer, lift enhancement and wave drag reduction. Of these local flow control techniques using energy deposition, this study is contributed to reduce the wave drag of supersonic flight. So far, various approaches to develop drag reduction technologies have been explored since the beginning of high-speed aerodynamics in order to realize economic supersonic flight. Aerodynamic drag force can be classified into friction and pressure drag. Friction drag is determined entirely by state boundary layer, and does not change greatly between subsonic and supersonic flight. However, pressure drag rapidly increases near the transonic flow due to shock wave generated by the aircraft body. Drag force due to shock wave is called wave drag. Shock waves have been a detriment for the development of supersonic aircrafts, which have to overcome high wave drag and surface heating from additional friction. The design for high-speed aircraft tends to choose slender shapes to reduce the drag and cooling requirements. Although this profile is adequate for fighter planes and missiles, it becomes engineering tradeoff between volumetric and fuel consumption efficiencies. In particular, this tradeoff significantly increases at the operating condition of commercial supersonic aircraft, which is preferred to be widebody capable of carrying hundreds of people. Since structure change of flight body for drag reduction reaches the limit, possibility of energy deposition technique was proposed to modify further aerodynamic performance. The intensive plasma generated by laser beam focusing is useful to control the supersonic flow field. When the laser energy is deposited into the oblique shock wave, blast waves and residual hot-spot interacts with bow shock wave in front of supersonic flight. Thereafter, plasma is transmitted to shock wave, and vortex is generated by well known baroclinic effects. This energy deposition technique has received much attention recently, and related investigations have been much conducted. Knight 1 characterizes the energy deposition scheme by using a deposited energy, pulse duration and pulse interval in respective dimensionless forms. If the pulse interval is long enough, flow after a pulse energy deposition is independent from previous pulses. Several numerical studies were conducted to examine the drag reduction due to energy deposition over bluntbody. Riggins et al. 2 investigated the drag reduction with the help of a computational fluid dynamics method using 2-D, axisymmetric Navier-Stokes equations. According to their study, drag force was reduced to value as low as

Aerodynamic Entropy Generation Rate in a Boundary Layer With High Free Stream Turbulence

An experimental-based model for prediction of flow noise with drag reducing polymers

Particle image velocimetry was employed to investigate the impact of convergent–divergent riblets on turbulent boundary layers in both clear water and liquid–solid two-phase flow fields containing 155 μm polystyrene particles. The turbulence statistics such as turbulence intensity and Reynolds stress were investigated. The spatial topology of spanwise vortex head and the development and evolution process of hairpin vortices were explored from Euler and Lagrange perspectives, respectively. Additionally, the particle distribution, concentration, and dispersion within the turbulent boundary layer were statistically analyzed. The results indicated that the boundary layer thickness, friction resistance, integrated turbulence intensity, and Reynolds stress were significantly lower on divergent riblet walls compared to convergent riblet walls. Notably, divergent riblets with a yaw angle of 30° exhibited the best drag reduction effect in both single-phase and two-phase flow fields. The addition of particles resulted in an increase in boundary layer thickness but effectively reduced turbulent fluctuations in the logarithmic region, enhancing drag reduction. This extended the drag reduction range of divergent riblets to a yaw angle of 45°, increasing the maximum drag reduction rate to 26.18%. Through spatial multi-scale local average structure function and finite-time Lyapunov exponent field analysis, it was found that the 30° divergent riblet wall significantly inhibited the development of vortex structures and reduced momentum exchange within the boundary layer. Conversely, the 30° convergent riblet wall had the opposite effect, while the particle phase inhibited the development of all wall turbulent structures. Analysis of particle concentration variations within different regions of the turbulent boundary layer revealed that as the normal height of the boundary layer increased, particle concentration gradually increased, and particle dispersion decreased accordingly. The analysis further showed that particle dispersion was mainly influenced by flow structures, whereas concentration was significantly affected by turbulence intensity. These findings elucidate the effect of the flow field on the particle phase and provide insights into the interaction mechanism between the flow field and particles.

The drag reduction in a zero pressure gradient (ZPG) turbulent boundary layer (TBL) using a rigid rodlike polymer was experimentally and numerically investigated. Using injection of the rigid polysaccharide scleroglucan, drag reductions of approximately 10–15 % were observed, with three distinct drag reduction regimes: a non-Newtonian flow region near the injector, followed by a region of nearly constant drag reduction, and finally a region of negligible drag reduction. Decreasing the effective rotary Peclét number reduced the drag reduction effectiveness. Increasing the concentration did not improve the drag reduction, but instead shifted the spatial development of the drag reduction further downstream. A complementary direct numerical simulation of the ZPG TBL using the rigid rod constitutive equation was performed at a matching inlet Reynolds number. The simulation assumed a homogeneous concentration distribution and used estimated effective parameters for the rodlike additive. Spatial evolution of the fiber stresses is rapid and develops asynchronously with the flow structure. The simulated turbulence statistics and experimental measurements at a position 23 boundary layer thicknesses downstream compare favorably, with the primary differences due to the concentration distribution assumed in the simulation.

Direct numerical simulation (DNS) is used to investigate the turbulent flat-plate boundary layer with localized micro-blowing. The 32 × 32 array of micro-holes is arranged in a staggered pattern on the solid wall, located in the developed turbulent region. The porosity of the porous wall is 23%, and the blowing fraction is 0.0015. The Reynolds number based on the inflow velocity is set to be 50,000. The structures of the turbulent boundary layer are carefully analyzed to understand the effects of micro-blowing and its drag reduction mechanism. The DNS results show that the drag reduction is efficient, and the local maximum rate of drag reduction achieves 40%. A low-speed “turbulent spot” near the micro-blowing region thickens the boundary layer. Some turbulent properties, such as the mean velocity profile, stream-wise vorticity and stream-wise velocity fluctuation are lifted up. Particularly, the tilting term of vorticity transport is significantly increased. Meanwhile, the visualization of 3-dimensional vortex displays several concave marks on the surface of the near-wall vortices, which is caused by the micro-jets, leading to more broken vortices and isotropic small scales. This impact travels downstream with a small distance due to the accumulation of the micro-jets, while the uplift effect will gradually disappear. In addition, FIK identity reveals that the spatial development term and mean wall-normal convection term play opposite roles in the contribution to the skin friction drag.

The turbulent boundary layer (TBL) over the hull surface of a water vehicle significantly elevates the drag force on the water vehicle. In this regard, effectively controlling the TBL can lead to a drag reduction (DR) effect and therefore improve the energy efficiency of water transportation. Many DR methods have demonstrated promising DR effects but face challenges in implementation at the scale of engineering application. In this regard, the recently developed dynamic free-slip surface method can resolve some of the critical challenges. It employs an array of freely oscillating air–water interfaces to manipulate the TBL and can achieve a substantial DR effect under certain control conditions. However, the optimal setting of the control parameters that would maximize the DR effect remains unclear. To answer these questions, this study systematically investigates the effects of multiple control parameters for the first time, including the geometric size and curvature of the interface, the frequency of active oscillation, and the Reynolds number of TBL. Digital Particle Image Velocimetry was used to non-invasively measure the velocity and vorticity field of the TBL, and the Charted Clauser method was used to calculate the DR effect. The presented results suggest that the oscillating free-slip interfaces reduce the flow velocity near the wall boundary and lift the transverse vorticity (and the viscous shear stress) away from the wall. In addition, the shape factor of the TBL is elevated by the oscillating interfaces and slowly relaxes back in the downstream regions, which implies a partial relaminarization process induced in the TBL. Up to 36% DR effect was achieved within the current scope range of the control parameters. All of the results consistently suggest that a large DR effect is achieved when the free-slip interfaces oscillate with large Weber numbers. These discoveries shed light on the underlying DR mechanism and provide guidance for the future development of an effective drag control technique based on the dynamic free-slip surface method.

Polymer drag reduction, diffusion and degradation in a high-Reynolds-number turbulent boundary layer (TBL) flow were investigated. The TBL developed on a flat plate at free-stream speeds up to 20ms−1. Measurements were acquired up to 10.7m downstream of the leading edge, yielding downstream-distance-based Reynolds numbers up to 220 million. The test model surface was hydraulically smooth or fully rough. Flow diagnostics included local skin friction, near-wall polymer concentration, boundary layer sampling and rheological analysis of polymer solution samples. Skin-friction data revealed that the presence of surface roughness can produce a local increase in drag reduction near the injection location (compared with the flow over a smooth surface) because of enhanced mixing. However, the roughness ultimately led to a significant decrease in drag reduction with increasing speed and downstream distance. At the highest speed tested (20ms−1) no drag reduction was discernible at the first measurement location (0.56m downstream of injection), even at the highest polymer injection flux (10 times the flux of fluid in the near-wall region). Increased polymer degradation rates and polymer mixing were shown to be the contributing factors to the loss of drag reduction. Rheological analysis of liquid drawn from the TBL revealed that flow-induced polymer degradation by chain scission was often substantial. The inferred polymer molecular weight was successfully scaled with the local wall shear rate and residence time in the TBL. This scaling revealed an exponential decay that asymptotes to a finite (steady-state) molecular weight. The importance of the residence time to the scaling indicates that while individual polymer chains are stretched and ruptured on a relatively short time scale (~10−3s), because of the low percentage of individual chains stretched at any instant in time, a relatively long time period (~0.1s) is required to observe changes in the mean molecular weight. This scaling also indicates that most previous TBL studies would have observed minimal influence from degradation due to insufficient residence times.

Drag reduction through turbulent boundary layer control (TBLC) is an essential way to develop green aviation technologies. Compared with traditional approaches for drag reduction, turbulence drag reduction is a relatively new technology, particularly for skin friction drag reduction, and it is becoming a hotspot problem worldwide. This paper focuses on the research of micro fluidic-jet actuators used for outer-layer boundary layer control with high-performance computing (HPC). This study aims to reduce turbulent drag by reshaping the flow structure within the turbulent boundary layer. To ensure the calculation accuracy of the core region and reduce the consumption of computing resources, a zonal LES/RANS strategy and WMLES method are proposed to simulate the effects of fluidic-actuators for outer-layer boundary control, in which high-performance computing has to be involved. The studies are performed on the classical zero-gradient turbulent flat plate cases, in which three different control strategies named “W-control,” “V-control,” and “VW-control” are used and compared to study the effects of drag reduction under a low Reynolds number at Reτ = 470 and a higher Reynolds number at Reτ = 4700. The mechanism for drag reduction is analysed via a pre-multiplied spectral method and a parallel dynamic mode decomposition (DMD) method. The results show that the present approach can effectively simulate the outer-layer turbulent boundary control where the “V-control” with the fluidic-jet actuator array behaves well to achieve an average drag reduction (DR) rate of more than 5% for the high Reynolds number case of the flat plate boundary layer. The high Reynolds shear stress and turbulent kinetic energy distribution in the boundary layer region show an obvious uplift under the effects of actuators, which is the main mechanism for drag reduction.

Drag reduction experiments using boundary layer heating