About

Peretz Friedmann is a Professor Emeritus in the Department of Mechanical and Aerospace Engineering at UCLA Samueli School of Engineering. His research interests include helicopter and fixed-wing aeroelasticity, active control of helicopter vibration and noise, aeroelasticity and aerothermoelasticity of hypersonic vehicles, unsteady aerodynamics, and optimization with aeroelastic constraints. He also focuses on structural dynamics and the aeroelasticity of flapping wing micro air vehicles (MAVs). His work involves studying the complex interactions between aerodynamic forces and structural responses in various aerospace applications, contributing to advancements in vibration control, noise reduction, and the design of high-speed and bio-inspired flight systems.

Research topics

• Computer Science
• Physics
• Engineering
• Mathematics
• Mechanics
• Aerospace engineering
• Mathematical optimization
• Programming language
• Mathematics education
• Materials science
• Pedagogy
• World Wide Web
• Mechanical engineering
• Geometry
• Structural engineering
• Mathematical analysis

Selected publications

• Reduced-Order Modeling of Resolved Turbulent Flows for High-Speed Fluid-Structure Interaction Analysis

  2025-01-03

  articleSenior author

  This study presents a reduced-order model method capable of generating resolved turbulent boundary layer pressure fluctuation loads at a reduced computational cost for aeroelastic analysis. The model generates unsteady pressure loads over a deformed panel by decomposing turbulent boundary layer flow into temporal and spatial components. First, an unsteady pressure fluctuation history over a flat plate is reproduced by superimposing spectral proper orthogonal decomposition modes and frequencies, which are orthogonal in space and time. Subsequently, spatial corrections to the pressure fluctuation magnitudes of the reconstructed flow are implemented to account for regions of flow compression and expansion over the deformed panel. Two assumptions required to construct the reduced-order model are verified: (1) one-way coupling exists between turbulence and structural responses, in that the impact of the former on the latter is negligible, and (2) spatial turbulent pressure variation is a linear function of modal coordinates of structural deformation. The reduced-order model produces accurate spectral properties of pressure fluctuations, which is critical for accurate prediction of the excitation of structural modes; however, the model overpredicts the magnitude. The results enable high-fidelity unsteady aeroelastic simulations at a reduced computational cost.

  PublisherDOI

• Reduced-Order Modeling of Turbulent Flows for High-Speed Aerothermoelastic Analysis

  AIAA Journal · 2025-12-09 · 1 citations

  articleSenior author

  This work presents a reduced-order model that efficiently generates unsteady aerothermal loads due to turbulent boundary layers for high-speed aerothermoelastic analysis. This prediction capability enables high-fidelity aerothermoelastic and structural fatigue analyses over long durations with tractable computational costs. Unsteady pressures over deformed structures are modeled by decomposing a turbulent boundary layer into temporal and spatial components. An unsteady pressure history over an undeformed structure is first reconstructed by superimposing spectral proper orthogonal decomposition modes and frequencies. Subsequently, the reconstructed pressures receive spatial corrections to model regions of flow compression and expansion. Heat fluxes over deformed structures are generated using a superposition/interpolation method that applies spatial corrections to the heat fluxes of an undeformed structure. An assumption required to construct the reduced-order model is verified: spatial pressure fluctuations and heat fluxes are a linear function of the modal coordinates of structural deformations in the linear regime. The reduced-order model produces accurate spectral contents of pressure fluctuations, which is critical for accurate structural excitation prediction; however, the model slightly overpredicts the fluctuation magnitudes. Likewise, the model produces accurate steady heat flux loads. The results enable high-fidelity unsteady aerothermoelastic simulations at a computational cost reduction of five orders of magnitude.

  PublisherDOI

• Turbulence and Upstream Shock Wave-Boundary Layer Interaction Effects on Compliant Structures

  AIAA Journal · 2025-06-30

  articleSenior author

  This study examines the effects of upstream shock wave-boundary layer interaction on fluid-structure interaction and the interaction between turbulence and structural responses. The configuration examined is a panel located downstream of a [Formula: see text] compression ramp. First, the ramp is exposed to Mach 4 flow, and the frequency content of pressure fluctuations in the turbulent boundary layer upstream and downstream of the compression corner is compared. It is found that intermediate frequencies that may influence fluid-structure interaction propagate into the downstream boundary layer. Next, the coupling mechanism between turbulence and structural responses is studied. Changes in turbulence caused by structural deformations are examined, followed by an investigation into the aeroelastic effects of modified turbulence on a flexible panel. One-way coupling exists between turbulence and structural responses because structural deformations significantly affect turbulence properties, but variations in turbulence have minor effects on structural responses. Finally, the role of turbulence in fluid-structure interaction is determined. Pressure fluctuation histories of varying intensity are applied to panels of varying thickness. A threshold is established indicating when boundary layer turbulence causes moderate structural deformations. The findings of the paper provide guidelines for designing control surfaces of hypersonic vehicles subjected to turbulent shock wave-boundary layer interaction effects.

  PublisherDOI

• AB1228 TIME LAG FROM THE ONSET OF RAYNAUD’S PHENOMENON TO SYSTEMIC MANIFESTATIONS IN SYSTEMIC SCLEROSIS PATIENTS OF A RACIALLY DIVERSE POPULATION

  Annals of the Rheumatic Diseases · 2024-06-01

  article
  PublisherDOI

• Evaluation of Shock Wave-Boundary Layer Interaction Modeling Capabilities for Use in a Hypersonic Aerothermoelastic Framework

  2024-01-04 · 2 citations

  articleSenior author

  This study examines the ability of a computational fluid dynamics solver that employs adaptive mesh refinement and embedded boundaries to model turbulent shock wave-boundary layer interactions. Additionally, a basis is provided for constructing a reduced order model for use in an aerothermoelastic analysis framework. The configuration examined is a panel on an inclined surface. First, the flow solver is used to model Mach 2.9 flow over a 24° compression ramp. The boundary layer properties, pressure profiles, and shock oscillation frequency modeled by the solver are compared to Direct Numerical Simulation and experimental results. Next, a strategy for generating the reduced order model is outlines. It is found that the frequency component caused by the shock oscillation does not propagate into the boundary layer downstream of the interaction and that deformations of the panel cause variations in time-averaged pressure distribution and turbulence in the boundary layer. However, the change in turbulence does not significantly affect the aeroelastic response of the structure. These findings support the use of a reduced order model composed of flow solutions where turbulence is one-way coupled.

  PublisherDOI

• Structural Dynamics

  Cambridge University Press eBooks · 2023-02-23 · 7 citations

  book1st authorCorresponding

  Master the principles of structural dynamics with this comprehensive and self-contained textbook, with key theoretical concepts explained through real-world engineering applications. The theory of natural modes of vibration, the finite element method and the dynamic response of structures is balanced with practical applications to give students a thorough contextual understanding of the subject. Enhanced coverage of damping, rotating systems, and parametric excitation provides students with superior understanding of these essential topics. Examples and homework problems, closely linked to real-world applications, enrich and deepen student understanding. Curated mathematical appendices equip students with all the tools necessary to excel, without disrupting coverage of core topics. Containing all the material needed for a one- or two-semester course, and accompanied online by Matlab code, this authoritative textbook is the ideal introduction for graduate students in aerospace, mechanical and civil engineering.

  PublisherDOI

• Dynamic Response of Structures

  Cambridge University Press eBooks · 2023 · 17 citations

  1st authorCorresponding
  • Computer Science
  • Computer Science
  • Mathematics education

  Master the principles of structural dynamics with this comprehensive and self-contained textbook, with key theoretical concepts explained through real-world engineering applications. The theory of natural modes of vibration, the finite element method and the dynamic response of structures is balanced with practical applications to give students a thorough contextual understanding of the subject. Enhanced coverage of damping, rotating systems, and parametric excitation provides students with superior understanding of these essential topics. Examples and homework problems, closely linked to real-world applications, enrich and deepen student understanding. Curated mathematical appendices equip students with all the tools necessary to excel, without disrupting coverage of core topics. Containing all the material needed for a one- or two-semester course, and accompanied online by Matlab code, this authoritative textbook is the ideal introduction for graduate students in aerospace, mechanical and civil engineering.

  PublisherDOI

• A Combined Computational and Experimental Study of Active Flow Control for Vibration Reduction on Helicopter Rotors

  AIAA SCITECH 2022 Forum · 2022-01-03 · 1 citations

  article

  View Video Presentation: https://doi.org/10.2514/6.2022-0325.vid The vibration reduction capability of active flow control (AFC) jets installed on the blades of a helicopter rotor is examined using comprehensive aeroelastic simulations and wind tunnel experiments. The simulations represent a four-bladed hingeless rotor operating at several advance ratios in the range of 0.20 ≤ µ ≤ 0.35. The wind tunnel experiments are performed on a 2D airfoil section undergoing dynamic pitching oscillations, representing an approximation of the rotor blade vibrations. The aerodynamic loads are modified using spanwise arrays of fluidically switching air jets on the suction and pressure surfaces upstream of the airfoil’s trailing edge. Comparison of the aerodynamic loads obtained from 2D CFD simulations and wind tunnel experiments show good agreement and have revealed that the fluidic actuators employed in this study produce a level of control authority appropriate for rotor vibration reduction. The comprehensive aeroelastic rotorcraft simulations, which employ a CFD-based reduced-order model (ROM) of the aerodynamic loads induced by AFC, predict a consistent level of vibration reduction across a range of flight conditions ranging from normal cruise to high-speed dynamic stall conditions. The rotor performance penalty associated with vibration reduction is also calculated. The additional rotor power required increases with advance ratio, due to increased drag penalty associated with the fluidic actuation.

  PublisherDOI

• Vibration Reduction in Rotorcraft Using Closed-loop Active Flow Control

  Journal of the American Helicopter Society · 2022-01-19 · 2 citations

  articleSenior author

  The vibration reduction obtained using active flow control (AFC) implemented on helicopter rotor blades is examined using comprehensive aeroelastic simulations. The flow control device consists of two fluidic jet actuators installed near the trailing edge of the blades on both the pressure side and the suction side. Closed-loop control simulations based on the higher-harmonic control (HHC) algorithm are developed to account for the discrete operating characteristics of the AFC actuators, since each actuator is either on or off at a fixed jet strength. The sensitivity of the closed-loop vibration reduction to the actuation power available, enforced as a saturation limit in the HHC algorithm, is examined. Results demonstrate the effectiveness and control authority of AFC for vibration reduction: up to 83% reduction of the 4/rev vibratory hub loads is possible. The vibration reduction is comparable to that obtained using a mechanically operated microflap operating in a closed-loop mode. Finally, the effect of AFC on the overall rotor performance during closed-loop vibration control is determined. Results indicate that moderate levels of vibration reduction are possible, with minimal impact on the overall performance when low actuation power is used. When the actuation power is increased, a diminishing level of improvement in the vibration reduction is observed, and the cost to overall performance increases significantly.

  PublisherDOI

• Aeromechanics and Aeroelastic Stability of Coaxial Rotors

  Journal of Aircraft · 2021 · 19 citations

  Senior authorCorresponding
  • Computer Science
  • Mechanics
  • Engineering

  Coaxial rotor aeroelasticity is complex due to the counter-rotating wake system, rotor lift offset, periodic blade passage loads, unsteady rotor wake interactions, reduced rotor speed, and stiff hingeless blades. In this study, the aeroelastic stability of a coaxial rotor is examined in hover and forward flight. The rotor wake is modeled with the viscous vortex particle method, a grid-free approach for calculating vortex interactions over long distances. The spanwise blade aerodynamic loading is calculated using a computational-fluid-dynamics-based reduced-order model in attached flow, and the ONERA dynamic stall model in separated flow. Two propulsive trim procedures are developed: one with the propulsor not operating, and the other with the vehicle at level attitude. An aeroelastic stability analysis based on Floquet theory is applied to the periodic system. A novel graphical method is developed to identify coupling between blade modes of the two rotors. The effects of lift offset and advance ratio on the hub loads, inflow distribution, and aeroelastic stability are examined to provide an improved physical understanding of the aeroelastic interactions. Results indicate that the blade passage effect is caused by the bound-circulation-induced inflow. The first and second lag modes are the least stable modes in hover and forward flight.

  PublisherDOI

Frequent coauthors

• Jack J. McNamara

  The Ohio State University

  53 shared

• Bryan Glaz

  DEVCOM Army Research Laboratory

  28 shared

• Nicolas Lamorte

  25 shared

• Ashwani K. Padthe

  23 shared

• Biju J. Thuruthimattam

  University of Michigan–Ann Arbor

  21 shared

• Kenneth G. Powell

  University of Michigan–Ann Arbor

  20 shared

• Andrew Crowell

  Virginia Commonwealth University

  20 shared

• Adam Culler

  Sierra Lobo (United States)

  19 shared