Hamid Johari

Bio: Hamid Johari is an academic researcher from California State University. The author has contributed to research in topics: Vortex & Jet (fluid). The author has an hindex of 23, co-authored 111 publications receiving 1967 citations. Previous affiliations of Hamid Johari include California State University, Northridge & United States Department of the Army.

Effects of Leading-Edge Protuberances on Airfoil Performance

Abstract: Lift, drag, and pitching moments of airfoils with leading-edge sinusoidal protuberances were measured in a water tunnel and compared with those of a baseline 63 4 -021 airfoil. The amplitude of the leading-edge protuberances ranged from 2.5 to 12% of the mean chord length; the spanwise wavelengths were 25 and 50% of the mean chord length. These ranges correspond to the morphology found on the leading edge of humpback whales' flippers. Flow visualization using tufts was also performed to examine the separation characteristics of the airfoils. For angles of attack less than the baseline stall angle, lift reduction and drag increase were observed for the modified foils. Above this angle, lift of the modified foils was up to 50% greater than the baseline foil with little or no drag penalty. The amplitude of the protuberances had a distinct effect on the performance of the airfoils, whereas the wavelength had little. Flow visualization indicated separated flow originating primarily from the troughs and attached flow on the peaks of the protuberances at angles beyond the stall angle of the baseline foil.

Penetration and Mixing of Fully Modulated Turbulent Jets in Crossflow

Abstract: Fully-modulated, incompressible, turbulent transverse jets were studied experimentally over a range of pulsing frequencies, duty-cycles, and at two jet-to-crossflow velocity ratios. The jet flow was completely modulated by operating a solenoid valve resulting in the shut off of jet supply during a portion of the cycle. The planar laserinduced fluorescence technique was used to determine the penetration, dilution, and structural features of the pulsed jets. The molecular mixing rate was quantified through a chemical reaction between the jet and crossflow fluids. Short injection times resulted in creation of vortex ring structures whereas long injection times produced axially elongated turbulent puffs, similar to a segment of the steady jet. The latter case resulted in only modest enhancement of the jet penetration depth and dilution. Pulsed jets dominated by vortex ring had penetration depths significantly greater than a steady jet with the same velocity ratio. Penetration of up to about 5 times the steady jet value at 50 jet diameters downstream of the jet exit was observed with 200 ms pulses. Duty-cycle had a significant effect on the performance of pulsed jets with short injection times. Increasing the duty-cycle for a fixed injection time diminished the jet penetration. The dilution and mixing rates of pulsed jets with short injection time were also increased over the steady jet. The greatest reduction in the mixing rate was approximately 50% for well-separated pulses with short injection times.

Scaling of Fully Pulsed Jets in Crossflow

Abstract: There is recent interest in the dynamics of pulsed jets in uniform crossflow. A classification scheme for fully pulsed Jets in crossflow is proposed based on the stroke ratio of the jet pulse and the duty cycle of the pulse train. Scaling relations for penetration and dilution of fully pulsed jets'in crossflow are derived from the self-similar scaling of turbulent vortex rings and puffs in quiescent media. Individual turbulent structures are assumed to drift freely with the crossflow in the proposed scaling. The penetration of fully pulsed jets scales with the fourth root of the velocity ratio, stroke ratio, and axial distance. The decay of mean concentration scales with the velocity ratio and axial distance to the -3/4th power. These scaling relations apply to distinct turbulent structures before any interaction between successive structures. The criteria for interaction among flow structures near the nozzle and in the far field are also presented.

Aerodynamic Characteristics of Finite Span Wings with Leading-Edge Protuberances

Abstract: A series of water-tunnel experiments were conducted to determine the effect of sinusoidal leading-edge protuberances on the aerodynamic characteristics of finite span wings. The models consisted of seven rectangular planform wings, two swept-leading-edge wings, and two wings with a planform resembling humpback-whale flippers. All models had an underlying NACA 634-021 profile with protuberance amplitudes of 0.025–0.12 times the chord length. The models were examined at Reynolds numbers up to 4.5×105 and angles of attack up to 30 deg. The lift and drag coefficients were nearly independent of Reynolds numbers above 3.6×105. Specific rectangular-planform models had appreciably greater lift coefficients over a limited angle-of-attack range when compared to the baseline model. However, with the exception of the planform that resembled the humpback-whale flipper, the lift-to-drag ratio of all leading-edge modified models was comparable to or less than the equivalent baseline model. The flipper model had a slight...

Boundary Layer Theory

Abstract: The boundary layer equations for plane, incompressible, and steady flow are $$\matrix{ {u{{\partial u} \over {\partial x}} + v{{\partial u} \over {\partial y}} = - {1 \over \varrho }{{\partial p} \over {\partial x}} + v{{{\partial ^2}u} \over {\partial {y^2}}},} \cr {0 = {{\partial p} \over {\partial y}},} \cr {{{\partial u} \over {\partial x}} + {{\partial v} \over {\partial y}} = 0.} \cr }$$

Conceptual models for partially premixed low-temperature diesel combustion

Abstract: Based on recent research within optically accessible engines and combustion chambers, conceptual models for low-temperature combustion (LTC) diesel engines are proposed. To provide a reference to which the LTC conceptual models may be compared, an established conceptual model framework for conventional diesel combustion is first reviewed and updated. Then, based on multiple optical diagnostic observations and homogeneous reactor simulations using detailed chemical kinetic mechanisms, extensions to the existing conceptual model are proposed. The LTC conceptual models are not intended to describe all LTC strategies, but rather a common subset of low-load, single-injection, partially premixed compression ignition conditions that are diluted by exhaust-gas recirculation to oxygen concentrations in the range of 10–15%. The models describe the spray formation, vaporization, mixing, ignition, and pollutant formation and destruction mechanisms that are consistent with experimental observations and modeling predictions for LTC diesel engines. Two separate subcategories are offered for either heavy-duty, large-bore or for light-duty, small-bore engines. Relative to the existing conventional diesel conceptual model, the features of the LTC conceptual models include longer liquid-fuel penetration, an extended ignition delay that allows more premixing of fuel, a more distinct and temporally extended two-stage ignition, more spatially uniform second-stage ignition, reduced and altered soot formation regions, and increased overmixing leading to incomplete combustion.

Particle Image Velocimetry of Human Cough

Abstract: Cough generated infectious aerosols are of interest while developing strategies for the mitigation of disease risks ranging from the common cold to SARS. In this work, the velocity field of human cough was measured using particle image velocimetry (PIV). The project subjects (total 29) coughed into an enclosure seeded with stage fog. Cough flow velocity profiles, average widths of the cough jet, and maximum cough velocities were measured. Maximum cough velocities ranged from 1.5 m/s to 28.8 m/s. The average width of all coughs ranged between 35 to 45 mm. Wide variability in the data suggests that future cough simulations consider a range of conditions.

Transverse jets and their control

Abstract: The jet in crossflow or transverse jet has been studied extensively because of its relevance to a wide variety of flows in technological systems, including fuel or dilution air injection in gas turbine engines, thrust vector control for high speed airbreathing and rocket vehicles, and exhaust plumes from power plants. These widespread applications have led over the past 50+ years to experimental, theoretical, and numerical examinations of this fundamental flowfield, with and without a combustion reaction, and with single or multi-phase flow. The complexities in this flowfield, whether the jet is introduced flush with respect to the injection wall or from an elevated pipe or nozzle, present challenges in accurately interrogating, analyzing, and simulating important jet features. This review article provides a background on these studies and applications as well as detailed features of the transverse jet, and mechanisms for its control via active means. Promising future directions for the understanding, interrogation, simulation, and control of transverse jet flows are also identified and discussed.