Heat transfer surface enhancement through the use of longitudinal vortices: a review of recent progress

A pulsed-plasma jet actuator is used to control the unsteady motion of the separation shock of a shock wave/boundary layer interaction formed by a compression ramp in a Mach 3 flow. The actuator is based on a plasma-generated synthetic jet and is configured as an array of three jets that can be injected normal to the cross-flow, pitched, or pitched and skewed. The typical peak jet exit velocity of the actuators is about 300 m/s and the pulsing frequencies are a few kilohertz. A study of the interaction between the pulsed-plasma jets and the shock/boundary layer interaction was performed in a time-resolved manner using 10 kHz schlieren imaging. When the actuator, pulsed at StL ≈ 0.04 (f = 2 kHz), was injected into the upstream boundary layer, the separation shock responded to the plasma jet by executing a rapid upstream motion followed by a gradual downstream recovery motion. Schlieren movies of the interaction showed that the separation shock unsteadiness was locked to the pulsing frequency of the actuator, with amplitude of about one boundary layer thickness. Wall-pressure measurements made under the intermittent region showed about a 30% decrease in the overall magnitude of the pressure fluctuations in the low-frequency band associated with unsteady large-scale motion of the separated flow. Furthermore, by increasing the pulsing frequency to 3.3 kHz, the amplitude of the separation shock oscillation was reduced to less than half the boundary layer thickness. Investigation into the effect of the actuator location on the shock wave/boundary layer interaction (SWBLI) showed qualitatively and quantitatively that the actuator placed upstream of the separation shock caused significant modification to the SWBLI unsteadiness, whereas injection from inside the separation bubble did not cause a noticeable effect.

Control of unsteadiness of a shock wave/turbulent boundary layer interaction by using a pulsed-plasma-jet actuator

Digital particle image thermometry/velocimetry (DPIT/V) is a relatively new methodology that allows for measurements of simultaneous temperature and velocity within a two-dimensional domain, using thermochromic liquid crystal tracer particles as the temperature and velocity sensors. Extensive research has been carried out over recent years that have allowed the methodology and its implementation to grow and evolve. While there have been several reviews on the topic of liquid crystal thermometry (Moffat in Exp Therm Fluid Sci 3:14–32, 1990; Baughn in Int J Heat Fluid Flow 16:365–375, 1995; Roberts and East in J Spacecr Rockets 33:761–768, 1996; Wozniak et al. in Appl Sci Res 56:145–156, 1996; Behle et al. in Appl Sci Res 56:113–143, 1996; Stasiek in Heat Mass Transf 33:27–39, 1997; Stasiek and Kowalewski in Opto Electron Rev 10:1–10, 2002; Stasiek et al. in Opt Laser Technol 38:243–256, 2006; Smith et al. in Exp Fluids 30:190–201, 2001; Kowalewski et al. in Springer handbook of experimental fluid mechanics, 1st edn. Springer, Berlin, pp 487–561, 2007), the focus of the present review is to provide a relevant discussion of liquid crystals pertinent to DPIT/V. This includes a background on liquid crystals and color theory, a discussion of experimental setup parameters, a description of the methodology’s most recent advances and processing methods affecting temperature measurements, and finally an explanation of its various implementations and applications.

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Digital particle image thermometry/velocimetry: a review

Control of a decelerating boundary layer. Part 3: Optimization of round jets vortex generators

Anexperimentalinvestigationoftheinteractionbetweena syntheticjetactuatorarrayand aturbulentboundary layer (Reµ =1:79 £ 10 3) is reported. The array consisted of three adjacent, parallel synthetic jet actuators, each with a rectangular orie ce having an aspect ratio of 45. The effect of the orientation of the jet orie ce, relative to the mean freestream direction, on thejet/boundary-layerinteraction wasinvestigated. The steady, spatially stationary featuresofthee owe eld wereexaminedusing hot-wireanemometry.Twoapparently differente owe eldswerefound in the boundary layer downstream of the actuator array. When the orie ce was normal to the mean e ow direction, the boundary layer was characterized by a wakelike region in the lee of the jet due to a blockage effect. When the orie ce was aligned with themean e ow direction, the e ow structurewasconsistent with the presenceof longitudinal vortices embedded in the boundary layer.

Interaction of a Synthetic Jet with a Crossflow Boundary Layer

A study was carried out of longitudinal vortices in a low speed turbulent boundary layer due to small jets. The jet was pitched at 45 deg and skewed between 0 and 120 deg. The effects of jet velocity ratio (0≤λ j ≤3.0) and skew angle (0≤θ≤120 deg) were addressed. Mass-averaged Navier-Stokes equations were solved using computational fluid dynamics and heat transfer. Turbulence closure was achieved using k-e model. Three types of vortex production were noted at various jet velocity ratios

Flow and heat transfer in a turbulent boundary layer through skewed and pitched jets

Measurements were made on the mean flowfield created through the interaction between a two-dimensional flat plate turbulent boundary layer on a flat plate and an inclined jet. The jet was generated by a nozzle of rectangular exit and was pitched and skewed to the oncoming flow. A total of three pitch angles and two jet velocity ratios were tested. The measurements were performed with a three-component laser Doppler anemometer system in a low speed wind tunnel. The dominant feature of the flow physics is a single longitudinal vortex produced as a result of jet/boundary layer interaction, with additional induced secondary features in the near-wall area in the spanwise direction. A salient feature of the flow is its maximum vorticity position which is located underneath the center of the vortex. Within the measured flow parameter range, the vortex development differs also from that of a round jet, differences in the velocity distribution are observed. The study provides contributions to flow physics.

Measurements of a longitudinal vortex generated by a rectangular jet in a turbulent boundary layer

The nearfield evolution of a longitudinal vortex embedded in a turbulent boundary layer was examined in a wind-tunnel test. The vortex was produced by an inclined round jet, the diameter of the jet being 0.4δ 0.99 . Flow properties were measured at a freestream velocity of 20 m/s using a laser Doppler anemometer. Control parameters are jet skew and pitch angles, and speed ratio. It was observed that a single vortex is generated at x = 10D. The effects of the control parameters on the strength and location of the vortex were examined. For an embedded vortex and favorable velocity distribution, the test results suggested a skew angle of 60 deg, a pitch angle of α = 30 deg, and speed ratios of γ = 1.0. The study provided insight into the flow physics and a database

Nearfield Evolution of a Longitudinal Vortex Generated by an Inclined Jet in a Turbulent Boundary Layer

Experimental and Numerical Analysis of Convective Heat Transfer in Turbulent Channel Flow with Square and Circular Columns

Numerical techniques for the calculation of velocity and temperature distributions in heated ducts have proved accurate but expensive in computer time and capacity. It is worth investigating to what extent simplification is possible without loss of accuracy. Entry-length heat transfer to upward laminar flow with combined convection in a vertical tube is taken as typical. Comparison is made between measured values and, first, a full numerical solution for constant thermophysical properties (viscosity and thermal diffusivity), secondly, the same solution but allowing for their individual and combined variation with temperature and, thirdly, a solution which assumes a series of truncated versions of the fully developed temperature distribution to establish corresponding velocity profiles, allowing for temperature-dependent properties.

M.W. Collins

Papers

Flow and heat transfer in a turbulent boundary layer through skewed and pitched jets

Measurements of a longitudinal vortex generated by a rectangular jet in a turbulent boundary layer

Nearfield Evolution of a Longitudinal Vortex Generated by an Inclined Jet in a Turbulent Boundary Layer

Experimental and Numerical Analysis of Convective Heat Transfer in Turbulent Channel Flow with Square and Circular Columns

Computational Methods for Entry Length Heat Transfer by Combined Laminar Convection in Vertical Tubes