DPM Physical Models Panel in ANSYS Fluent 2024 R1

#+CATEGORY: cfd

#+TAGS: DPM Fluent physical models forces thermophoretic

#+MATURITY: growing

DPM Physical Models :: Complete Panel Walkthrough for ANSYS Fluent 2024 R1

This page walks through every toggle and setting in the Fluent DPM Physical Models tab. Each option is explained with its governing physics and when it matters for PM2.5 aerosols. Target application: shisha simulation.

Accessing the Physical Models Tab

Models > Discrete Phase > Edit... > Physical Models tab

This tab controls:

• Which forces act on the particle (beyond drag and gravity)
• Whether heat/mass transfer is computed
• Whether particle-phase radiation is included
• Turbulence dispersion settings

Forces Section

Gravity (Body Force)

• Toggle: Body Forces checkbox
• Default: On
• Direction: defined in Operating Conditions or Cell Zone Conditions

Gravity always acts. For PM2.5 the gravitational settling velocity is negligible compared to turbulent diffusion (Stokes number << 1), but include it in the setup for completeness -- the cost is zero.

Saffman Lift Force

• Toggle: Saffman lift force checkbox
• Computes the lift force on a sphere in a shear flow:

: F_lift = 6.46 * mu * (rho_p / rho_f)^0.5 * d_p^2 * (omega x (u_p - u_f)) / (nu * Re_omega)^0.5

• Relevant for: large particles (> 10 \mu m) in strong shear layers
• For PM2.5: negligible. Saffman force scales with d_p^2, which is 10^-12 for 1 \mu m particles. You can leave it off or on -- either way the contribution is subdominant.

Virtual Mass Force

• Toggle: Virtual mass checkbox
• Accounts for the added mass effect: accelerating a particle also accelerates the surrounding fluid

: F_vm = -C_vm * rho_f * V_p * (du_p/dt - du_f/dt)

where C_vm = 0.5 for a sphere.

• Relevant for: bubbles in a liquid (rho_p < rho_f) or strongly accelerating flows
• For PM2.5 (solid aerosol in air): negligible. rho_p / rho_f ~ 1000, so the virtual-mass term is ~10^-3 of the drag.

Pressure Gradient Force

• Toggle: Pressure gradient force checkbox
• Force due to the pressure gradient in the flow:

: F_pg = -V_p * grad(p)

• Relevant for: bubbles, low-density particles in strong pressure gradients
• For PM2.5 in shisha: negligible. The pressure gradient across the chamber is small, and particle density is high relative to air.

Thermophoretic Force

• Toggle: Thermophoretic force checkbox
• Drives particles from hot to cold regions

: F_th = -D_T * grad(T) / T

where D_T is the thermophoretic coefficient (depends on particle size, thermal conductivities of gas and particle).

• Relevant for: combustion, hot exhaust, smoke cooling
• For shisha PM2.5: potentially significant. Smoke enters at ~350 K and cools toward ambient (~300 K). The temperature gradient near walls can drive sub-micron particles toward the glass. Enable this if you solve the energy equation and expect temperature gradients > ~10 K/cm.

How to compute thermophoretic force for specific particle sizes: Thermophoretic velocity scales approximately as: : v_th ~ (k_g / rho_p / d_p) * (grad(T) / T)

For d = 1 \mu m, grad(T) = 100 K/m, k_g = 0.026 W/mK, rho_p = 1200 kg/m^3: : v_th ~ 0.026 / (1200 * 1e-6) * (100 / 300) ~ 7 mm/s

Compare to typical flow velocity ~0.1 m/s: thermophoretic drift is ~7% of the flow. Marginal but not zero. Include it.

Heat and Mass Transfer Section

Enable heat transfer

• Toggle: Heat transfer checkbox
• When enabled, Fluent tracks the particle temperature and computes convective heat transfer using the Ranz-Marshall correlation

For inert PM2.5: enable this if you want any temperature change on the particle. The heating time constant is ~4 ms for 1 \mu m particles, so they reach thermal equilibrium with the local gas very quickly.

Evaporation (Law 2) / Boiling (Law 3) / Wet combustion

• These laws are only active for Droplet or Multicomponent particle types
• For Inert particles (shisha smoke): /all evaporation/boiling laws are inactive/ regardless of toggle state

Radiation

• Toggle: Radiation Interaction checkbox
• Computes radiative heat exchange between particles and surrounding surfaces/medium
• For PM2.5: negligible at shisha temperatures. Smoke particles are too small to emit/absorb significant radiation compared to convective exchange.

Coupled heat transfer

• Toggle: Coupled heat transfer checkbox (only with two-way coupling)
• Enables the heat source term from particles back to the continuous phase
• For one-way coupling (shisha PM2.5): not applicable

Turbulence Dispersion Section

Stochastic Tracking (DRW)

• Toggle: Stochastic tracking radio button
• Activates the Discrete Random Walk model
• Parameters:
  • Number of Tries: how many stochastic trajectories per parcel (default = 1)
  • Eddy lifetime model: default (eddy lifetime based on k-epsilon) or =TKE-based=/ dissipation-based

This is mandatory for PM2.5. Without stochastic tracking, all particles follow the mean flow and there is zero turbulent dispersion. With DRW, the turbulent velocity fluctuations are sampled, and the particle cloud spreads realistically.

See also: DPM Turbulence Interaction for the full DRW algorithm.

Cloud Model

• Toggle: Cloud model radio button (alternative to stochastic tracking)
• Represents the particle cloud as a Gaussian distribution (no individual trajectories)
• Used for: spray combustion where you need ensemble statistics without tracking every parcel
• For PM2.5: not recommended. The cloud model smears out the particle distribution; stochastic tracking gives pointwise trajectory information you need for concentration fields.

Additional Forces (User-Defined)

DEFINE\_DPM\_BODY\_FORCE

• Add a user-defined body force to the particle equation of motion
• This is the way to add Brownian motion for PM2.5

Shisha PM2.5 configuration:

• Enable Thermophoretic force
• Enable Stochastic tracking with Number of tries = 10
• Write a UDF for Brownian force using DEFINE_DPM_BODY_FORCE
• Leave Saffman, Virtual mass, Pressure gradient unchecked (they are negligible but cost almost nothing; you can leave them on for completeness)

Full Physical-Models Checklist for Shisha PM2.5

--- :PROPERTIES: :seeAlso: cfd_dpm-setup-activation, cfd_dpm-turbulence-interaction, cfd_dpm-heat-mass-transfer, cfd_dpm-brownian, cfd_dpm-shisha-pm25 :END: