Generation Of Streamwise Vortices Research Articles

On-Site Characterization of Automotive Diffuser Aerodynamics by 3D LPT

This investigation proposes a novel LPT facility featuring Helium-Filled Soap Bubbles flow tracers, LED illumination and two high-speed cameras to characterize the dominating flow patterns within automotive underbodies. A remote control (RC) car model, fitted with custom-made floor and diffusers, traverses a region of seeded air following the Ring of Fire methodology. Underground-placed cameras view the car through a transparent panel, providing unparalleled optical access to the underbody of the car. The on-site measurement setup and the interaction between car model and ground enhance the realism and fidelity of the experiments, while potentially reducing testing costs associated with wind tunnel operation. The setup is shown to be a valid alternative to conventional testing approaches to capture flow separation, 3D flow evolution and differences in the flow field between the four tested configurations, whereby the diffuser angle was varied in the range between 5° and 20°. The 15° diffuser led to the largest velocity and pressure peaks under the car, whereas the 10° diffuser produced the most downforce thanks to the diffuser “pumping” effect, leading to a large region of low pressure under the vehicle. Notably, the 20° diffuser featured the most prominent flow separation at the diffuser’s leading edge, heavily affecting its ability to sustain low pressures under the car. The results show that the wide tyres have a major impact on the underbody flow, because their large wakes induce mass flow leakage through the sides of the car, thus disrupting the mechanism of downforce generation and impairing the generation of streamwise vortices.

Turbulent wake characteristics for a circular cylinder in proximity to a moving wall

This study investigates the scenario of flow past a circular cylinder in proximity to a moving wall (or equally a body translating in still fluid parallel to a stationary wall). Fifty high-fidelity three-dimensional direct numerical simulations are performed over a parameter space of turbulent Reynolds numbers (Re) of 300 to 1000 combined with gap-to-diameter ratios (G/D) of 0.2 to 3. The flow, hydrodynamic and turbulence characteristics over the (Re, G/D) parameter space are examined in detail. Small-scale rib-like mode B structures and alternate vortex shedding are observed for all cases. The streamwise location for the vortex shedding (quantified by the wake recirculation length Lr) varies strongly with both Re and G/D. The variation of Lr with Re and G/D can be explained by the spanwise circulation $\varGamma_z$ fed into the wake, where the variation trends of Lr and $\varGamma_z$ are inversely correlated. The variations of the mean drag and fluctuating lift coefficients are also governed by the variations in Lr and $\varGamma_z$ . The total kinetic energy in the wake region reduces drastically as G/D reduces below 0.8, which is contributed collectively by (i) reduction in the strength of the shed vortices, (ii) downstream movement of the location of vortex shedding and (iii) associated delayed generation of streamwise vortices. The present results on a moving wall also help to explain several flow and hydrodynamic characteristics reported in the literature for a stationary wall, because the moving wall eliminates the complex wall boundary layer and retains a ‘clean’ near-wall effect.

Enhancing the noise reduction capability of serrations usinglow-profile vortex generators

This paper presents an experimental investigation on improving the noise reduction capability of the trailing edge serrations using vortex generators. Experiments were conducted on a 150 mm chord flat plate at four angle of attack between 0° and 9° and flow speeds of 20-50 m/s. The Reynolds number based on the chord length ranges from 2.1×105 to 5×105. Vane type vortex generators with various heights of h/δ = 0.16-0.25 were placed along with the serration roots, where δ is the boundary layer thickness at the trailing edge without the vortex generators. The aeroacoustic measurements were made with a phased microphone array, and the aerodynamic measurement near the trailing edge was measured using a hot-wire. Additionally, the lift and drag forces were measured by load cells. The source integrated noise spectra showed a slight reduction in the trailing edge noise over a broadband frequency range. The generation of streamwise vortices by vortex generators can be identified in the wake measurement. These streamwise vortices were further found to counteract the cross-flow generated at serration roots, hence improving the performance of the serration. Moreover, the presence of low-profile vortex generators was found to have an insignificant effect on the lift and drag coefficients.

Influence of coupling schemes of radial and circumferential flame stabilization modes on flow and combustion characteristics of compact combustion for gas turbine

The combination of the strut and cavity is a common structural configuration for ultra-compact combustor in the gas turbine. The streamwise vortices inside the cavity play the role of stabilizing the flame and mass exchange in the spanwise direction, which is very important for the compactness and ignition performance. In this paper, the numerical simulations and the experiments were performed to study the influence of the combustor strut blocking ratio, cavity length-to-depth ratio, and strut inclination angle on the flow, the streamwise vortices distribution, as well as the combustion characteristics. We also discussed the generation of streamwise vortices and the effects of streamwise vortices on the flow and combustion performance. The results showed that the strut blocking ratio and cavity length-to-depth ratio mainly affect the strut recirculation zone and the cavity recirculation zone, the interaction of which is the key to the generation of the streamwise vortices. The reasonable distribution of the streamwise vortices promotes the mass exchange and the uniformity of the temperature distribution. In addition, the strut inclination angle has a great influence on the flow characteristics, and the forward-inclined strut is more conducive to combustion. Based on the obtained conclusions, a new design of ultra-compact integrated combustor is proposed, and it has improved flow and combustion performance.

Geometric scale effect of the subsonic-supersonic shear layer based on a sinusoidal lobed splitter plate

Passive flow control tools have been widely used to enhance shear layer mixing in ramjet engines. Lobed mixer is an excellent method in mixing enhancement performance, while there is no comprehensive consensus on its mixing enhancement mechanism in a subsonic-supersonic shear layer. In this work, a sinusoidal lobed mixer is used to increase mixing between the subsonic and supersonic flows, and large eddy simulation (LES) is used to research the potential flow control mechanisms. Parameter experiments were carried out and the mixing performance was obtained. The numerical conclusions show good similarity with experimental conclusions. The impact of the geometric scales of the modified lobed mixer in the evolution of shear layer is investigated. It is found that sinusoidal lobed splitter plate with wavelength 10 mm and amplitude 2 mm shows the best mixing thickness experimentally and numerically, where compared with other cases, it is 3.3 times thicker than that of the flat plate. It is demonstrated that the sinusoidal lobed mixer performance is affected by its amplitude and wavelength. Large amplitude increases the initial disturbance, induces large scale streamwise vortex and enhances its mixing effectively. The number and size of large scale coherent vortices are inversely proportional to the wavelength. Smaller wavelength can accelerate the transformation process of the vortex structure, but it causes the slow development of shear layer downstream. Sinusoidal lobed mixer can promote the generation of large-scale streamwise vortices, significantly shorten the transition distance, and achieve better shear layer mixing.

Evaluation of tilt control for wind-turbine arrays in the atmospheric boundary layer

Abstract. Wake redirection is a promising approach designed to mitigate turbine–wake interactions which have a negative impact on the performance and lifetime of wind farms. It has recently been found that substantial power gains can be obtained by tilting the rotors of spanwise-periodic wind-turbine arrays. Rotor tilt is associated with the generation of coherent streamwise vortices which deflect wakes towards the ground and, by exploiting the vertical wind shear, replace them with higher-momentum fluid (high-speed streaks). The objective of this work is to evaluate power gains that can be obtained by tilting rotors in spanwise-periodic wind-turbine arrays immersed in the atmospheric boundary layer and, in particular, to analyze the influence of the rotor size on power gains in the case where the turbines emerge from the atmospheric surface layer. We show that, for the case of wind-aligned arrays, large power gains can be obtained for positive tilt angles of the order of 30∘. Power gains are substantially enhanced by operating tilted-rotor turbines at thrust coefficients higher than in the reference configuration. These power gains initially increase with the rotor size reaching a maximum for rotor diameters of the order of 3.6 boundary layer momentum thicknesses (for the considered cases) and decrease for larger sizes. Maximum power gains are obtained for wind-turbine spanwise spacings which are very similar to those of large-scale and very-large-scale streaky motions which are naturally amplified in turbulent boundary layers. These results are all congruent with the findings of previous investigations of passive control of canonical boundary layers for drag-reduction applications where high-speed streaks replaced wakes of spanwise-periodic rows of wall-mounted roughness elements.

Flow control of a D-shaped bluff body using different DBD plasma actuators

The control performance of a streamwise-oriented dielectric barrier discharge (DBD) plasma actuator, a set of plasma streamwise vortex generators (PSVGs), and a hybrid actuator of the plasma actuator and PSVGs on the reduction in bluff body flow separation, vortex-induced vibration (VIV), and wake fluctuation is experimentally investigated. Experiments are conducted in a low-speed and low-turbulence wind tunnel with a Reynolds number between 3 ×103 and 1.2 ×104 based on the diameter of a half circular cylinder. Particle image velocimetry (PIV) is used to obtain details on the flow fields over a short D-shaped bluff body. Force measurement is conducted to compare the reduction in drag and vibration oscillations using these three types of plasma actuators. The PIV flow fields show that all of the plasma actuators suppress the flow separation on the bluff body, narrow the size of the wake, and decrease the turbulence kinetic energy (TKE) level in the wake. This stable controlled vortex shedding system can reduce the effect of the natural frequency of the bending stiffness-dominated cylinder structure system, thus avoiding the occurrence of resonance in advance. The reduction in drag and lateral lift oscillation are studied by mapping the changes in force coefficients and fluctuations as a function of Reynolds number. A comparison of these plasma actuators shows that the hybrid actuator achieves best drag reduction, suppression of lift oscillation, and Kármán vortex shedding in the wake at low speed, because three-dimensional flow structures are generated on the surface of the bluff body that consequently enhance the mixing. The results suggest that PSVGs and ameliorative actuators are promising for wake flow control in bluff bodies at low speeds.

Enhanced thermal performance in tubular heat exchanger contained with V-shaped baffles

The vortex-flow device is a promising streamwise vortex generator employed to produce the counter-rotating vortices along a heated tube to augment the heat transfer rate with comparatively smaller friction loss penalty. Thus, the influences of V-shaped baffle vortex generator (hereafter called “V-baffle”) insert in a heated tube on thermal-performance enhancement were experimentally examined in the current work. The geometric characteristics of the V-baffles mounted repeatedly on the edges of a flat plate/tape were three ratios of relative baffle blockages, (b/D = BR = 0.1, 0.15 and 0.2), and four ratios of baffle pitches, (P/D = PR = 0.5, 1.0, 1.5 and 2.0) at a fixed angle of attack (α = 30°). The present V-baffle placed on the tape edge was aimed to lessen the pressure loss from disturbing the central core flow for the case of placing it on the double sides of a tape as found in the literature. The experiment was conducted by letting air flow through the test tube with Reynolds number (Re) in the range of 4192 to 25,750. The current study indicated that the friction factor and heat transfer using the V-baffle inserts increase considerably with rising BR but reducing PR. The V-baffle with BR = 0.2, PR = 0.5 provides the highest friction factor and rate of heat transfer at about 18.25 and 4.46 times above the plain tube, respectively. A new modified thermal enhancement factor (TEF) is offered and found that its peak for each case appearing at the lowest Re, is in a range of 2.14–2.34 where the optimum TEF of 2.34 is visible at PR = 1.0, BR = 0.15. Furthermore, correlations of Nusselt number and friction factor for the present V-baffles are determined. TEF of the current device is found to be superior to that of other enhanced devices in comparison.

Experimental Investigation of Vortex-Tube Streamwise-Vorticity Characteristics and Interaction Effects with a Finite-Aspect-Ratio Wing

An experimental study is conducted to analyze a streamwise-oriented vortex and investigate the unsteady interaction with a finite-aspect-ratio wing. A pressurized vortex tube is used to generate streamwise vortices in a wind tunnel and the resulting flow behavior is analyzed. The vortex tube, operated at various pressures, yields flows that evolve downstream under several freestream wind tunnel speeds. Flow measurements are performed using two- and three- dimensional (2D and 3D) particle image velocimetry to observe vortices and their freestream interactions from which velocity and vorticity data are comparatively analyzed. Results indicate that vortex velocity greater than freestream flow velocity is a primary factor in maintaining vortex structures further downstream, while increased supply pressure and reduced freestream velocity also reduce vortex dissipation rate. The generated streamwise-oriented vortex is also impinged on a finite-aspect-ratio airfoil wing with a cross-section of standard NACA0012 airfoil. The wingtip-aligned vortex is shown to investigate the interaction of the streamwise vortex and the wingtip vortex region. The results indicate that the vorticity at the high vortex-tube pressure has a significant effect on the boundary layer of airfoil.

Effects of Low-Frequency Noise in Crossflow Transition Control

Direct numerical simulations were performed to investigate crossflow transition control by the three-dimensional steady upstream flow deformation technique under the influence of low-frequency fluctuations. In wind-tunnel or flight experiments with discrete roughness elements, the fluctuations most likely arise from the interaction of the elements with background turbulence or base-flow fluctuations. If plasma actuators are applied instead, they directly enhance—unintended by discharge randomness—the low-frequency fluctuation level of the flow, degrading the control effectiveness that is based on inducing steady control vortices. To elucidate the fundamental mechanisms responsible for the promoted transition due to low-frequency fluctuations, the present work focused on localized volume forcing, modeling two actuator configurations, that is, plasma virtual roughness elements and streamwise vortex generators. The noise is introduced by modulating the steady volume force using a broadband low-frequency signal. Even when the steady control mode clearly dominates over low-frequency traveling disturbances, transition can still be significantly accelerated by the known secondarily amplified type III modes. Compared with the much localized active or passive roughness elements investigated in previous crossflow experiments, elongated vortex generators spreading over a larger streamwise distance diminish the receptivity of misaligned traveling modes and thus offer a better control performance.

Study on the behavior of streamwise vortices formed between leading edge tubercles in a compressor cascade

A study has been carried out to investigate the formation mechanism and development of streamwise vortices induced by leading edge tubercles in a high speed compressor cascade. The preliminary assessment of the cascade performance in terms of the total pressure loss coefficient shows that the loss reduction is achieved at high incidence angles. A smaller wavelength leads to higher additional losses at the design point, but gives rise to a greater loss reduction at high incidence angles. The W4 cascade achieves the maximum loss reduction of 36.1% at i = 10°, as well as the stall angle improvement of 27.6%. The formation mechanism of streamwise vortices is elaborated on the basis of the streamwise vorticity equation, in which the streamwise turning terms may be responsible for the generation of streamwise vortices. Slices of streamwise vorticity at various streamwise locations, combined with vorticity strength distributions, have been presented to study the development of streamwise vortices. The counter-rotating vortices are divided into the crest-induced streamwise vortices (CSVs) and trough-induced streamwise vortices (TSVs). A streamwise vortex pair formed from part of the CSV sheets behind troughs, is gradually entrained by the TSV pair along the streamwise direction. In addition, the tubercles with a smaller wavelength result in higher streamwise vorticity strength, with which the streamwise vortices interact with the flow separation more sufficiently and delay the separation to a greater extent.

Flow field characteristics of a supersonic jet influenced by downstream microjet fluidic injection

Mixing characteristics of a supersonic jet influenced by a Downstream Microjet Fluidic Injection (DMFI) system are numerically investigated. The DMFI system is built on the observation of a previous experimental study that utilized transverse fluidic injection from four equally spaced injection ports placed on an injection tube at a distance downstream of a 1.5 Mach number nozzle. The measurements from these previous experiments demonstrated thickening and mixing enhancement of the jet shear layer as a result of fluidic injection. The current numerical study examines the underlying physics of the flow field, as well as the effectiveness of the DMFI system at smaller mass flow rate ratios compared to those utilized in the previous experiments. Results indicate good agreement with the trend observed by the experimental study, and considerable improvement in enhancement of the jet mixing is observed. The observed mixing enhancement is attributed to the presence of the microjet tube and fluidic injection and the consequent generation of streamwise vortices, as well as the natural separating bow shock due to the transverse flow injection. DMFI system is shown to enhance the early mixing, resulting in attenuation of the downstream turbulence production. Furthermore, the DMFI system is demonstrated to be an effective method of mixing enhancement for supersonic jets with a potential for reducing the jet noise radiation.

Flow Control on a Bluff Body Using Dielectric Barrier Discharge Plasma Actuators

In this study, a traditional dielectric barrier discharge (DBD) plasma actuator and plasma streamwise vortex generators (PSVGs) are installed on a bluff body to control the wake fluctuations. Experiments are conducted in a wind tunnel with a chord Reynolds number of 5000. Particle image velocimetry and hot-wire measurements are used to obtain the details of the flowfields. The results show that both DBD and PSVG actuations clearly reduce the size of the recirculation bubble regions, and they significantly suppress the wake fluctuations. A comparison of the traditional DBD and PSVG cases shows that PSVGs have better control performance on the reduction of the recirculation bubble sizes and the Karman vortex shedding frequency in the wake at low speed. Specifically, the PSVGs have a strong three-dimensional effect on the entire flowfield and generate streamwise vorticities that interact with the shear flow over the bluff body. The local flow separations are thereby suppressed considerably. The induced streamwise vorticities are further entrained downstream by the ambient freestream, which consequently reduces the recirculation bubble behind the bluff body and suppresses the wake fluctuations. These results suggest that PSVGs are promising devices for conducting flow control in bluff body wakes at low speed.

Experimental investigation of plasma vortex generator in flow control

PurposeThe purpose of this paper is to compare the effects of two different configurations of plasma streamwise vortex generators (PSVG), including comb-type and mesh-type in controlling flow. This is demonstrated on the NACA 0012 airfoil.Design/methodology/approachThe investigations have been done experimentally at the various electric and aerodynamic conditions. The surface oil flow visualization method has been used to the better understanding of the flow physics and the interaction of the oncoming flow passing over the airfoil and the vortex generated by comb-type PSVG.FindingsThis paper demonstrates the potential capabilities of the mesh-type and comb-type PSVGs in controlling flow in unsteady operation. It was found that the vortex generated by the mesh-type PSVG in unsteady operation was an order of magnitude stronger than comb-type PSVG. The flow visualisation technic proved that only a part of the plasma actuator is effective in the condition that the actuator is installed only on a portion of the upper surface of the airfoil.Originality/valueThis paper experimentally confirms the capabilities of the mesh-type PSVG unsteady operation in compare with comb-type PSVG in controlling flow, whereby recommends using mesh-type PSVG in the leading edge in front of comb-type PSVG on the entire wingspan to prevent the stall.

Wake control using spanwise-varying vortex generators on bridge decks: A computational study

A passive control device called spanwise-varying vortex generator (VG) is proposed to mitigate vortex shedding of a bridge model, since the 3D spanwise-varying control method shows high efficiency in suppressing vortex shedding behind bluff bodies. Pairs of VGs were arranged on the lower surface of the bridge in the spanwise direction to generate streamwise vortices and perturb two-dimensional spanwise vortex shedding. Delayed detached-eddy simulation and dynamic mode decomposition were carried out to examine their effects on wake control. The results demonstrate that with VG control, the regular spanwise vortices are highly distorted and are accompanied by a spanwise mismatch in the vortex shedding process. Consequently, the regular spanwise vortex shedding is substantially suppressed, resulting in a considerable reduction in vortical strength in the wake and significant increases in the vortex formation length. In addition, the dynamic mode decomposition reveals that VGs can redistribute the energy spectrum and transport the energy from the mode associated with the regular vortex shedding to other low-frequency modes. Therefore, forces induced by vortex shedding are considerably reduced.

Passive flow control application for rotorcraft in transonic conditions

PurposeHelicopter rotor blades are usually aerodynamically limited by the severe conditions present in every revolution: strong shock wave boundary layer interaction on the advancing side and dynamic stall on the retreating side. Therefore, different flow control strategies might be applied to improve the aerodynamic performance.Design/methodology/approachThe present research is focussed on the application of passive rod vortex generators (RVGs) to control the flow separation induced by strong shock waves on helicopter rotor blades. The formation and development in time of the streamwise vortices are also investigated for a channel flow.FindingsThe proposed RVGs are able to generate streamwise vortices as strong as the well-known air-jet vortex generators. It has been demonstrated a faster vortex formation for the rod type. Therefore, this flow control device is preferred for applications in which a quick vortex formation is required. Besides, RVGs were implemented on helicopters rotor blades improving their aerodynamic performance (ratio thrust/power consumption).Originality/valueA new type of vortex generator (rod) has been investigated in several configurations (channel flow and rotor blades).

Understanding liquid-jet atomization cascades via vortex dynamics

Temporal instabilities of a planar liquid jet are studied using direct numerical simulation (DNS) of the incompressible Navier–Stokes equations with level-set (LS) and volume-of-fluid (VoF) surface tracking methods. $\unicode[STIX]{x1D706}_{2}$ contours are used to relate the vortex dynamics to the surface dynamics at different stages of the jet breakup – namely, lobe formation, lobe perforation, ligament formation, stretching and tearing. Three distinct breakup mechanisms are identified in the primary breakup, which are well categorized on the parameter space of gas Weber number ($We_{g}$) versus liquid Reynolds number ($Re_{l}$). These mechanisms are analysed here from a vortex dynamics perspective. Vortex dynamics explains the hairpin formation, and the interaction between the hairpins and the Kelvin–Helmholtz (KH) roller explains the perforation of the lobes, which is attributed to the streamwise overlapping of two oppositely oriented hairpin vortices on top and bottom of the lobe. The formation of corrugations on the lobe front edge at high $Re_{l}$ is also related to the location and structure of the hairpins with respect to the KH vortex. The lobe perforation and corrugation formation are inhibited at low $Re_{l}$ and low $We_{g}$ due to the high surface tension and viscous forces, which damp the small-scale corrugations and resist hole formation. Streamwise vorticity generation – resulting in three-dimensional instabilities – is mainly caused by vortex stretching and baroclinic torque at high and low density ratios, respectively. Generation of streamwise vortices and their interaction with spanwise vortices produce the liquid structures seen at various flow conditions. Understanding the liquid sheet breakup and the related vortex dynamics are crucial for controlling the droplet-size distribution in primary atomization.

Experimental study on mitigating vortex-induced vibration of a bridge by using passive vortex generators

It has been reported that three-dimensional spanwise-varying control is very efficient in suppressing vortex shedding behind bluff bodies. Therefore, a new passive control method, spanwise-varying passive vortex generator (PVG), was proposed to mitigate the vortex-induced-vibration (VIV) of a bridge. The PVGs can directly generate streamwise vortices, which behave like secondary wake instability modes (3D instability modes). Therefore, the PVGs can cause 3D instability of spanwise vortex shedding and suppress VIV. In this study, isolated PVGs were arranged on the lower surface of the bridge in spanwise direction, and their control effects on mitigating VIV were investigated through wind tunnel tests. The results show that as PVGs are arranged with spanwise distance of 1–3H (H is the height of the bridge) and the PVG height is 0.1–0.25H, both vertical and torsional VIV can be completely suppressed, and the trailing edge is the key region to suppress VIV. Further, the control effects of PVG are barely influenced by the boundary layer of the bridge. The spectrum analysis of the wake velocity shows that the PVG can completely suppress the vortex shedding near a wake, and thereby resulting in the vanishing of VIV.

Parameter influence on the evolution of low-aspect-ratio rectangular synthetic jets

The evolution of low-aspect-ratio rectangular synthetic jets is investigated with dye and laser-induced-fluorescence (LIF) flow visualization techniques. This paper analyzes the impacts of three key parameters on the evolution of vortical structures, respectively, including orifice aspect-ratio (AR), dimensionless stroke length, and Reynolds number. Compared with circular synthetic jet, all the rectangular synthetic jets display vortex ring axis switching, and may develop into two types of stream-wise vortices I and II. The evolution of the vortex ring and the generation of stream-wise vortices are influenced by three parameters. In particular, stream-wise vortices are not detected for a low AR, stroke length or Reynolds number case.

The logarithmic and power law behaviors of the accelerating, turbulent thermal boundary layer

Direct numerical simulation of spatially evolving thermal turbulent boundary layers with strong favorable pressure gradient (FPG) shows that the thermal fluctuation intensity, θ′+ and the Reynolds shear stress, u′v′¯+ exhibit a logarithmic behavior spanning the meso-layer (e.g., 50≤y+≤170). However, the mean thermal profile is not logarithmic even in the zero pressure gradient (ZPG) region; instead, it follows a power law. The maxima of u′2¯+ and v′θ′¯+ change little with the strength of acceleration, while v′+, w′+, and u′v′¯+ continue to decay in the flow direction. Furthermore, θ′+ and u′θ′¯+ surprisingly experience changes from constants in ZPG to sharp rises in the FPG region. Such behavior appears to be due to squashing of the streaks which decreases the streak flank angle below the critical value for “transient growth” generation of streamwise vortices, shutting down production [W. Schoppa and F. Hussain, “Coherent structure generation near-wall turbulence,” J. Fluid Mech. 453, 57–108 (2002)]. The streamwise vortices near the wall, although shrink because of stretching, simultaneously, also become weaker as the structures are progressively pushed farther down to the more viscous region near the wall. While the vortical structures decay rapidly in accelerating flows, the thermal field does not—nullifying the myth that both the thermal and velocity fields are similar.