Hamid Johari

Effects of acceleration on turbulent jets

Effects of acceleration on turbulent jets were investigated in a series of flow visualization experiments. Prior to the initiation of acceleration, a steady jet with a Reynolds number of 3000 was established. Three distinct acceleration schemes of linear, quadratic, and exponential were utilized to increase the nozzle exit velocity by an order of magnitude. As the flow accelerated, a discernible ‘‘front’’ was established. The parcels constituting the front were less diluted than the steady jet parcels at the same location. For each acceleration scheme, the temporal evolution of the front position had the same functional form as the nozzle velocity. The front velocity increased linearly with the acceleration rate for the linear and quadratic cases. In comparison with a steady jet, the front’s lateral growth rate was reduced by 16% in the linear case and by 25% in the two nonlinear cases, even though the linear cases had generally larger acceleration rates. A model, based on the scaling of centerline veloci...

Vortex shedding in the near wake of a parachute canopy

The dynamics of flexible parachute canopies and vortex shedding in their near wake are studied experimentally in a water tunnel. The velocity field was measured by particle image velocimetry for two different canopy diameters. The periodic oscillation of the canopy diameter about a mean value which is referred to as ‘breathing’ has a non-dimensional frequency, based on the free-stream velocity and the mean canopy projected diameter, of approximately 0.55 for the range of Reynolds numbers examined. The dimensionless breathing frequency observed in the experiments is consistent with the values for larger canopies. The shear layer emanating from the canopy rolls up and sheds symmetric vortex rings. The frequency of vortex shedding was measured to be the same as the canopy breathing frequency. This Strouhal number is unique in the sense that it is much higher than those associated with rigid axisymmetric bluff bodies such as disks and spheres. The canopy breathing is shown to stem from the cyclical variation of suction pressure, resulting from the passage of vortex rings, on the exterior surface of the canopy. The added mass associated with the breathing of the canopy is found to be accountable for up to 40 % of the canopy drag fluctuations in the range of parameters investigated.

Flame Length Measurements of Burning Fuel Puffs

Abstract An experimental study of burning of natural gas puffs generated by a fully modulated jet, has been conducted. The present study concentrated on the effects of duty-cycle and puff volume, or equivalently the injection time, on the flame length. It is observed that when individual puffs are well-separated, considerably shorter flame lengths, by as much as a factor of four, are obtained compared to a steady jet flame. Furthermore, the flame length of individual puffs scales with the initial volume of puff, in agreement with the previous studies of buoyant puffs in aqueous media. For a given puff volume, as the duty-cycle is increased beyond a given value, the flame length tends to rise rapidly due to the interaction among neighboring puffs. A dimensionless parameter is found which indicates the transition of puff characteristics.

Structure and Flame Length of Fully-Modulated, Turbulent Diffusion Flames

The turbulent flame structure and flame length of fully-modulated diffusion flames was examined over a range of pulsing frequencies, injection flow rates, and duty-cycles. An injection system employing an electronically-controlled solenoid value was used to discharge puffs of unhealed natural gas and ethylene fuel into still air at one atmosphere pressure. Video imaging of the luminosity from the sooting regions of the flame revealed two distinct types of flame structure. For small injected volumes and short injection times, compact, puff-like structures with a short flame length were generated. More elongated “cigar-shaped” structures, with a longer flame length closer to that of steady-state flames, resulted from longer injection times and larger injected volumes. An injection parameter which characterizes the transition from puff-like to cigar-shaped flame behavior is presented. For puffs, an increase in duty-cycle generally led to an increase in flame length. This increase was less for cigar-shaped flames. The downstream location of the puff-like flame structures increased roughly with time to the 1/2 power, in agreement with buoyant thermals and starting jets.

Computation of Flow over a High-Performance Parafoil Canopy

The incompressible Navier-Stokes equations together with the one equation Spalart-Allmaras turbulence model were solved using a finite volume flow solver to examine the flowfield and forces on the central cross section of a high-performance parafoil canopy. The parafoil surfaces were assumed to be impermeable, rigid, and smooth. The flowfield consists of a vortex inside the parafoil cell opening, which effectively closes off the opening and diverts the flow around the leading edge. The flow about the parafoil experiences a rather bluff leading edge, in contrast to the smooth leading of the baseline airfoil. A separation bubble exists on the lip of the parafoil lower surface. The lift coefficient of the parafoil section increases linearly with the angle of attack up to 8.5 degrees and the parafoil lift-curve slope is about 8% smaller than the same for the baseline airfoil. The cell opening has a major effect on the drag before stall; the parafoil drag is at least twice the baseline airfoil drag. The minimum drag of the parafoil section occurs over the angle-of-attack range of 2.5-7 degrees.

Near wake of an impulsively started disk

The near wake of an impulsively started disk was studied computationally by a finite element code with a Smagorinsky turbulence model. The shear layer separating from the disk lip rolled up into a symmetric starting vortex ring at first. As time evolved, the vortex stretched in the downstream direction and flow instabilities caused the vortex ring to become wavy eventually leading to the breakup of the ring. The complete breakup and shedding of the starting vortex ring took a time of approximately 14D/U, where D is the disk diameter and U is the freestream velocity. The starting vortex ring circulation attained a plateau of ≈2.6UD at a time of about 4D/U, in good agreement with the experimental findings by Balligand (2000). The radial circulation profiles are Gaussian during the symmetric phase and collapse together at a time of 4D/U. Beyond this time, the vortex ring celerity is constant and vorticity extends to the symmetry axis. The base pressure coefficient becomes positive as the vortex ring moves aw...

CHEMICALLY REACTIVE TURBULENT VORTEX RINGS

Employing an aqueous acid‐base reaction, the minimum mixing rate of turbulent vortex rings was investigated in a water tank. Vortex rings were generated by a simple apparatus with a cylindrical geometry. The released fluid surrounding the vortex core mixed very rapidly when compared with the fluid in the toroidal core. Moreover, the fluid within the core did not mix uniformly in the azimuthal direction. The normalized distance a vortex ring must travel, in order to completely mix with the ambient fluid to a specific volumetric ratio, depends on the aspect ratio of the generating cylinder. Scaling arguments are presented that relate the above distance to the spreading rate and the generating apparatus parameters. Due to the very small net entrainment rate of vortex rings, the detrainment of core material cannot be ignored when the mixing rate of the core is considered.

Ultrasonic Method for Aircraft Wake Vortex Detection

This paper describes the experimental proof of concept study for an ultrasonic method of wake vortex detection. This new acoustic method uses travel time of acoustic pulses around a closed path to measure the net circulation within the acoustic path. In this application the closed path encloses the vorticity shed from one side of a Piper PA-28 aircraft wing. Magnitude and sign of circulation detected is comparable to the expected circulation generated by the Piper PA-28 test aircraft. This study demonstrates the validity of the acoustic method in detecting aircraft wake vortices. Further investigations and applications using this technique are discussed.

Computation of Flow over a High Performance Parafoil

The steady, incompressible Navier-Stokes equations along with the one equation SpalartAllmaras turbulence model were solved using a finite-volume flow solver to examine the flow field and forces on the central cross-section of a high performance parafoil. The parafoil surfaces were assumed to be impermeable, rigid and smooth. The parafoil flow field consists of a vortex just inside of the cell opening which effectively closes off the cell and diverts the flow around the leading edge. The flow about the parafoil experiences a rather bluff leading edge in contrast to the smooth leading of the baseline airfoil. The flow pattern also creates a separation bubble on the lower leading edge of the parafoil. The lift of the parafoil section increases linearly with the angle of attack up to 8.5 and the parafoil lift curve slope is about 8% smaller than the same for the baseline airfoil. The cell opening has a major effect on the drag prior to stall; the parafoil drag is at least twice the baseline airfoil drag. The minimum drag of the parafoil section occurs over the angle of attack range of 2.5 – 7.

CO/UNBURNED HYDROCARBON EMISSIONS OF STRONGLY-PULSED TURBULENT DIFFUSION FLAMES

The CO and unburned hydrocarbon (UHC) emissions of pulsed turbulent diffusion flames were examined by injecting unheated ethylene into a combustor with an air coflow at atmospheric pressure. In all cases the flames were fully modulated (fuel flow fully shut off between injection intervals). Video imaging was performed and time-averaged emissions were measured on the combustor centerline. For short injection times ( ≤ 46 ms), compact, puff-like structures were generated. Longer injection times produced elongated flame structures with flame lengths closer to that of steady flames. The highest emission indices of CO and UHC were found for compact, isolated puffs. The emissions for all flames approached the steady-flame levels for a duty cycle (jet-on fraction) of approximately 0.4. This suggests that there are combinations of injection time and duty cycle for fully modulated, turbulent diffusion flames that produce emissions comparable to the steady flame but with a significantly shorter flame length.