Reduction In Turbulence Intensity Research Articles

The groove effect on wake characteristics of rotating cylinders

In the present study, active and passive flow control methods have been implemented to investigate their effects on the wake flow structures of a circular cylinder. Grooves having circular, rectangular, and triangular cross sections have been applied to the cylinders exposed to the rotation rates, α, from 0 to 0.79. The experiments have been conducted by particle image velocimetry at a Reynolds number of Re = 5 × 103. The contour graphics of time-averaged results have been presented. Moreover, the variations in velocity profiles have also been depicted. The experimental results revealed significant variations for flow patterns, wake structures, and turbulence parameters due to the effects of both groove geometries and rotational motion. In the stationary cases, for turbulence intensity, the circular grooved cylinder exhibited a 15% increase, while the triangular grooved cylinder showed a slightly higher increase of around 20% compared to that of the bare cylinder (BC). Conversely, in non-stationary cases, the rectangular grooved cylinder displayed the most prominent reduction in turbulence intensity, decreasing by approximately 10% compared to that of the BC. The groove type has considerably affected the flow structures of the wake regions, especially for the lower rotation rates.

Wire mesh fences for manipulation of turbulence energy spectrum

Manipulation of turbulence within an atmospheric boundary layer flow by application of woven wire mesh fences is investigated. Turbulence properties behind fences of different porosities and mesh opening widths were determined from velocity measurements in a wind tunnel. It is found that with the application of a fence with a porosity of 0.46, the streamwise turbulence intensity can be reduced from the inflow level of 12.5%–8.8% and the integral length scale can be reduced from 380 to 270 mm. The results show that behind the mesh fences turbulence kinetic energy decays as a power law function of the downstream distance for all wire mesh fences tested in the wind tunnel. The decay rate of turbulence kinetic energy is faster, and a larger reduction in the integral length scale is achieved for fences with porosities between 0.46 and 0.64 compared to higher porosities of between 0.73 and 0.75. Porosity of the woven wire meshes is found to be the key parameter which influences their turbulence reduction performance. In the end, application of the wire mesh fences for reduction of wind loads on solar panels and heliostats is discussed. Evaluation of wind loads based on the reduction of turbulence intensity and integral length scale shows that up to 48% and 53% reduction in peak drag and lift forces on a heliostat, respectively, can be achieved with application of mesh fences.

Large-eddy simulation of compressible turbulent flow in convergent-divergent nozzles with isothermal wall

We present results from high-order accurate large-eddy simulations of compressible flow through convergent-divergent nozzles with and without bell-shaped divergent portions. The nozzles have isothermal walls and circular cross-sections. The incoming flow is a turbulent, subsonic fully-developed pipe flow at M=0.37 and friction Reynolds number 214. A smooth transition from subsonic to supersonic flow occurs in the nozzles when the back pressure is kept small, accompanied by a reduction of turbulence intensities in the diverging portion of the nozzles. Strong reductions in production and pressure-strain correlations are observed as the flow undergoes acceleration and expansion. When the back pressure is high, the flow in the nozzle remains subsonic, accompanied by a reduction in turbulence in the convergent portion and an amplification in the divergent portion.

Attenuation of turbulence by the passive control of sweep events in a turbulent boundary layer using micro-cavities

Cavity arrays have been previously identified to disrupt the sweep events and consequently the bursting cycle in the boundary layer by capturing the structures responsible for the Reynolds stresses. In the present study, the sensitivity of a flushed-surface cavity array in reducing the turbulent energy production has been investigated. Two plates of varying thicknesses and four different backing cavity volumes were considered, at three different Reynolds numbers. The volume of the backing cavity was shown to be the most important characteristic in determining the attenuation of streamwise velocity fluctuations within the logarithmic region of the turbulent boundary layer. However, the results also demonstrated that the orifice length of the cavity array had negligible effect in modifying the reduction of the turbulent energy by the cavity array in this investigation. The results show that the maximum reduction in turbulence generation achieved for this study occurs when the backing volume is 3.1 × 106 times greater than the viscous length scale at Reθ = 3771. The reduction in turbulence intensity, sweep intensity, and energy spectrum were shown to be 5.6%, 6.3%, and 13.4%, respectively. By decreasing the cavity volume to zero, no change in the turbulent boundary layer turbulence statistics was found. The results suggest a larger reduction in turbulence intensity, sweep intensity, and energy spectrum that can be achieved with a larger backing volume.

Analysis of Separation Modes Variation in a Scramjet Combustor with Single-Side Expansion

Cold flow analysis is applied in a canonical scramjet combustor to obtain insights into key physics of the symmetric/asymmetric combustion modes under different equivalence ratios. Systematic experiments have been implemented in a single-expanding duct with backpressure produced by a cylinder at Mach 3. Fine structures of separated flow have been gained by the nanobased planar laser scattering system. High-frequency pressure signals are acquired to study the unsteady behaviors of separation shock. Velocity profiles obtained from particle image velocimetry are validated by numerical simulations. The results indicate that the symmetric backpressure itself can cause asymmetric separation. A steady symmetric separation mode forms when backpressure is relatively low. An unsteady asymmetric separation mode appears when backpressure is high enough. A large separated region always dominates at the expansion wall in the asymmetric separation mode. The asymmetric separation is formed by the increment of boundary-layer thickness and the reduction of turbulence intensity after expansion corner. An interlaced turbulence intensity distribution between the straight and expansion walls accounts for the transition of separation modes under different backpressure. A broadband oscillation of shock foot lies in the asymmetric separation mode, which is evolved from low-frequency unsteadiness in the shock-wave–boundary-layer interaction. Meanwhile, asymmetric backpressure is able to restrain the development of the separated region and forces the separation mode to stay symmetric and steady under high backpressure.

Damping of unwanted turbulence in wave–current experiments

Laboratory testing of structures placed in combined wave–current flows is a valuable source of information for the fulfillment of offshore engineering related tasks and the development of ocean energy devices. In recirculating wave–current flumes, one of the problems encountered during such experimental studies is the occurrence of undesirable current induced velocity fluctuations. These fluctuations often result in significant disturbances of the generated wave profiles. In this paper, a physical flow filter is introduced that significantly reduces fluctuations in the current profile while permitting wave passage. This is achieved by passing the wave–current flow through a setup of perforated net tubes that allows for both horizontal and vertical flow motions. An in depth investigation of the properties of different filter configurations is presented, focusing on the reduction of turbulence intensities in the flow field as well as the influence of the setup on waves. The filter characteristics are quantified in terms of its deflection, absorption, and transmission properties. It is shown that the overall setup effectively reduces velocity fluctuations, resulting in stable wave–current conditions and allowing for high quality laboratory testing.

An assessment of the impact of reduced averaging time on small wind turbine power curves, energy capture predictions and turbulence intensity measurements

The effect of varying the averaging time of measured data used to calculate wind turbine power curves is examined. The effects of reducing the averaging time from 10 minutes to 1 minute, as recommended for small wind turbines are investigated using power performance data recorded using a 15kW wind turbine. Test site data have been processed according to IEC 61400-12-1 to provide power curves and annual energy yield predictions [1]. A number of issues are explored: the systematic distortion of the power curve that occurs as averaging time is decreased, the errors introduced by the use of one minute averaged power curves to calculate energy yield and the reduction of turbulence intensity as averaging time is reduced. Recommendations for improved small wind turbine testing and energy yield calculation are given.

Dynamic Pressure Fluctuations in Stepped Three-Side Spillway

One type of outlet works in dams are Three-side spillways that despite of their hydraulic limitations and construction problems, they were being selected, in storage dams, as best option under specific topographical conditions. Considerable energy losses and great turbulence are hydraulic characteristic of these spillways. Hydraulic performance with targeting to reduce pressure fluctuations in side channel is an important issue in this type of spillways design. In this study, effect of stepping of three-side spillway's Ogee profile on the dynamic pressure fluctuation have been investigated using finite volume method and RNG turbulence model. The turbulence intensity as a dimensionless number was used for quantitative study of dynamic pressure fluctuations. The results showed that the proposed form of ogee profile caused a significant reduction in turbulence intensity within the side channel. On the other hand, the stepped Ogee profiles of three-side spillways caused to simple construction and ease of operation.

Effects of an axisymmetric contraction on a turbulent pipe flow

AbstractThe flow in an axisymmetric contraction fitted to a fully developed pipe flow is experimentally and numerically studied. The reduction in turbulence intensity in the core region of the flow is discussed on the basis of the budgets for the various turbulent stresses as they develop downstream. The contraction generates a corresponding increase in energy in the near-wall region, where the sources for energy production are quite different and of opposite sign compared to the core region, where these effects are caused primarily by vortex stretching. The vortices in the pipe become aligned with the flow as the stretching develops through the contraction. Vortices which originally have a spanwise component in the pipe are stretched into pairs of counter-rotating vortices which become disconnected and aligned with the mean flow. The structures originating in the pipe which are inclined at an angle with respect to the wall are rotated towards the local mean streamlines. In the very near-wall region and the central part of the contraction the flow tends towards two-component turbulence, but these structures are different. The streamwise and azimuthal stresses are dominant in the near-wall region, while the lateral components dominate in the central part of the flow. The two regions are separated by a rather thin region where the flow is almost isotropic.

A study on some effects of a drag-reducing agent on the performance of a stirred vessel

The influence of the addition of a drag reducing agent (100 ppm PAA) to a pure liquid (water) in a stirred vessel has been studied. The vessel was stirred with two types of agitators, a Rushton turbine and an axial A310 impeller. This study was meant to implement previous findings with new data and various types of measurements were performed to determine a number of parameters. It is confirmed that the drag-reducing agent decreases power draw and increases mixing time. Mean flow and the fluctuating velocities were determined (with the PIV technique) and notable reduction in turbulence intensity was detected. The mechanism of tracer dispersion (followed by the PLIF technique) was also documented qualitatively. Finally, the influence of the drag-reducing agent on the particle size distribution of the solid product of a precipitation reaction (reaction of barium chloride and sodium sulphate) was shown.

Direct-simulation-based study of turbulent flow over various waving boundaries

We use direct numerical simulation of stress-driven turbulent Couette flows over waving surfaces to study turbulence in the vicinity of water waves. Mechanistic study is performed through systematic investigation of different wavy surface conditions including plane progressive Airy and Stokes waves with and without wind-induced surface drift, as well as stationary wavy walls and vertically waving walls for comparison. Two different wave steepness valuesak= 0.1 and 0.25 are considered, whereais the wave amplitude andkis the wavenumber. For effects of wave age, defined as the ratio between the wave phase speedcand the turbulence friction velocityu*, we consider three values, namelyc/u*= 2, 14 and 25, corresponding to slow, intermediate and fast waves, respectively. Detailed analysis of turbulence structure and statistics shows their dependence on the above-mentioned parameters. Our result agrees with previous measurement and simulation results and reveals many new features unreported in the literature. Over progressive waves, although no apparent flow separation is found in mean flow, considerable intermittent separations in instantaneous flow are detected in slow waves with large steepness. The near-surface coherent vortical structures are examined. We propose two conceptual vortex structure models: quasi-streamwise and reversed horseshoe vortices for slow waves and bent quasi-streamwise vortices for intermediate and fast waves. Detailed examination of Reynolds stress with quadrant analysis, turbulent kinetic energy (TKE) and TKE budget with a focus on production shows large variation with wave phase; analysis shows that the variation is highly dependent on wave age and wave nonlinearity. Comparison between Airy waves and Stokes waves indicates that although the nonlinearity of surface water waves is a high-order effect compared with the wave age and wave steepness, it still makes an appreciable difference to the turbulence structure. The effect of wave nonlinearity on surface pressure distribution causes substantial difference in the wave growth rate. Wind-induced surface drift can cause a phase shift in the downstream direction and a reduction in turbulence intensity; this effect is appreciable for slow waves but negligible for intermediate and fast waves. In addition to providing detailed information on the turbulence field in the vicinity of wave surfaces, the results obtained in this study suggest the importance of including wave dynamics in the study of wind–wave interaction.

Direct shortwave radiative forcing of the Asian dust aerosol on dust emission

The physical processes of the feedback mechanism of direct shortwave radiative forcing of the Asian dust aerosol on dust emission has been examined using simulated results with the coupled (with dust shortwave radiative forcing) and the non-coupled model (without dust shortwave radiative forcing) based on the MM5 model and the Asian Dust Aerosol Model on 19 March 2002. The results indicate that a significant dust emission reduction occurs in the high dust concentration (HDC) region of the dust source region whereas an enhanced dust emission appears in the downstream of the dust source region. It is found that Asian dust aerosols raised during the daytime by the strong surface wind cause negative shortwave radiative flux near the surface, which in turn reduces the sensible heat flux causing the cooling of the air, thereby enhancing stable stratification. The dynamic adjustment of the negative radiative flux of the dust induces a positive pressure anomaly over the HDC region and a negative pressure anomaly toward the synoptic low pressure center, resulting in a dipole shape of pressure anomaly field near the surface. The associated secondary circulation of this pressure anomaly together with the reduction of turbulent intensity due to the reduced sensible heat flux reduces the low-level wind speed thereby reducing dust emission in the upstream of the HDC region of the dust source region (Region I), while enhancing the low-level wind speed in the downstream region (Region II), which in turn enhances dust emission. This enhanced dust emission is smaller than the emission reduction in the upstream, resulting in overall dust emission reduction during the daytime.

Three-Dimensional Vortex Method for Gas-Particle Two-Phase Compound Round Jet

This paper proposes a three-dimensional vortex method for a gas-particle two-phase compound round jet. The method can take account of the interaction between the two phases by calculating the motion of particles and the behavior of gas vortex elements through the Lagrangian approach. In order to discuss the validity of the method, an air jet, loaded with small glass particles, issuing from a round nozzle into the co-flowing air stream is simulated. The simulation demonstrates that the air turbulence modulations due to the particles, such as the relaxation of velocity decay, the decrement of momentum diffusion in the radial direction at the fully developed region, and the reduction of turbulent intensity and Reynolds shear stress, are successfully captured by the method.

C220 波状面上における安定成層流の乱流構造

Large eddy simulation was performed for flow and heat transfer under stable density stratification in a wavy wall channel to reveal the effect of stratification on turbulent structures. Low-Mach-number approximation was applied to treat density variation. The iso-surface of instantaneous temperature in the middle of the channel shows that the effect of stable stratification is strong and greatly reduces turbulence there. On the other hand, instantaneous structures and two-point correlations near the wavy wall suggest that characteristic near-wall structures and their scales are not strongly affected by the extent of stratification in spite of considerable reduction of turbulent intensity.

The entrainment behavior of a turbulent axisymmetric jet in a viscous host fluid

Although turbulent jets have been studied extensively, one configuration that has not received much attention is the viscosity-stratified jet, wherein a turbulent jet of lower viscosity issues into a density-matched host liquid of higher viscosity. We present experimental data for scalar dispersion and two-dimensional velocity measurements in the axial plane of a turbulent axisymmetric jet with a Reynolds number (Re) of 2,000 issuing into a viscous host liquid at viscosity ratios (m) ranging from 1 to 55. The presence of a strong viscosity discontinuity across the jet edge results in a significant decrease in the scalar spread rate. We attribute this to the rapid reduction in turbulence intensity and the suppression of large engulfing eddies at the jet edge. The velocity profile, on the other hand, indicates that the velocity width and mass flux reduce with increasing m up to about 20, but then increase for higher values of m. This non-monotonic variation is explained by the growing influence of viscous stress for m>20. The scalar spread rate, the velocity spread rate, the centerline velocity decay rate, and the jet mass flux are all minimized for m≈20 for Re=2,000.

Effects of Turbulence by Ultrasonic Vibration on Fluid Flow in a Rectangular Channel

The effect of ultrasonic vibration in the Reynolds number (Re) range of 1500 to 6000 on fluid flow in a square channel was investigated experimentally. An ultrasonic transducer was fixed at the center of the bottom of a square test channel, and a standing wave field was formed in this channel. The velocity of the fluid was measured by a laser-Doppler velocimeter (LDV). By applying ultrasonic vibration to the laminar flow, agitation or disturbance of cavitation bubbles was produced in the flow and the transition to turbulent flow from laminar flow was promoted downstream. The ultrasonic vibration velocity near the wall is closely approximated by the expression of the log-law even if there exists a laminar flow region. In high Re regions, turbulence can be restrained by the ultrasonic vibration near the wall. Acoustic cavitation near the wall causes the reduction of turbulence intensity under turbulent regions. Since it is possible to control the fluid flow externally due to the easy transmission of ultrasonic vibration in liquids, this technique can be applied to fluid flow in various channels as a noncontact turbulence promoter.

STRUCTURE OF TURBULENT VELOCITY AND BUOYANCY FLUCTUATIONS IN STRATIFIED TURBULENT SHEAR FLOW

The effects of buoyancy on turbulence structure of a buoyant surface jet are investigated. Velocities and buoyancy fluctuations are simultaneously measured, and their statistic quantities are examined. Distributions of turbulence quantities are strongly affected by buoyancy. Their variations against the shear layer Richardson number can be explained in terms of the production terms in their transport equations. As the buoyancy effects increase, turbulence structure and buoyancy fluctuation balance change from a jet-like regime to a stratified flow-like one. This transition is well related with the large-scale coherent structure's dynamics peculiar to the turbulent shear flows. The buoyancy results in not only reduction of turbulence intensities but also diminution of turbulence scales.

Multitube turbojet noise‐suppression studies using cross‐correlation techniques

The noise‐producing region of a suppressed turbojet exhaust is studied by cross‐correlating static pressure fluctuations within the exhaust with farfield sound for Mach numbers up to Mj=0.99, using a 31‐tube nozzle having an area ratio Ar=3.1. Measurements made with an unsuppressed turbojet exhaust having an equivalent exit area and operating under effectively equal thrust levels serve as the experimental control for this work. Static pressure‐level measurements, made with a calibrated high‐temperature acoustically damped probe tube, show that noise suppression by multitube nozzles results from reduced turbulence levels. The maximum fluctuating static‐pressure level in the unsuppressed turbojet exhaust is typically 5–6 dB higher than static‐pressure levels in the suppressed exhaust under conditions of effectively equal static thrust. This suggests that the turbulence intensity in the multitube suppressor flow is reduced in excess of 20% compared with the unsuppressed jet exhaust. Static pressure fluctuation spectrums in the suppressor flow are peaked at the eddy passage frequency and a double spectral peak is found in the region where the suppressor jets coalesce. Estimates of the eddy‐scale‐length to jet‐diameter ratio made from measurements of local jet velocity and eddy passage frequency indicate that a reduction in eddy size occurs when neighboring suppressor tube flows are brought into close proximity to one another. Maximum normalized cross‐correlation coefficients in the unsuppressed turbojet exhaust of approximately cPF?0.2 are found for Mj=0.99 at an angle of 30° from the jet axis. Normalized cross‐correlation coefficients are significantly lower in the interior of the multitube suppressor flow. Measurements of the partially normalized cross‐correlation function show that sound radiated from within the hot, high‐speed suppressor flow is refracted by the flow from neighboring jets. This work demonstrates that the high‐frequency sound radiated directly to the farfield by a multitube suppressor nozzle is primarily from the exterior tubes and that the overall suppression of noise can be attributed to a reduction in turbulence intensity.

Aerodynamics and Noise of Coaxial Jets

The objective of this investigation was to develop a unified prediction method for estimating the aerodynamic and noise characteristics of jets issuing from nozzles of arbitrary geometric shapes. The method has been developed and demonstrated for dual-flow coaxial jets. An extension of Reichardt's theory is utilized to predict the mean velocity, temperature, and axial turbulence intensity distributions throughout the jet plume. The generic noise intensity spectrum is synthesized by a slice-of-jet approach, wherein each axial location in the plume contributes to the sound generation in one dominant frequency band. The propagation of the source spectrum to the far field is modeled by means of convected quadrupoles embedded in a parallel slug flow. Extensive predictions of the aeroacoustic characteristics of coaxial jets were made and compared with experiment. The agreement between theory and experiment is quite good, except at high frequencies and shallow angles to the jet axis, where refraction is overestimated. A major conclusion drawn from these results is that the noise reduction attained by a coaxial jet conies primarily from a reduction in turbulence intensity.