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Momentum ODE

Mathematical Modelling — Fluid Dynamics & Heat TransferContinuum mechanics; boundary-layer similarity theory

1

Symbol

The third-order similarity-reduced streamwise momentum equation; its formula is composed on demand from the Powell–Eyring, unsteady, magnetic, and buoyancy terms.

Main equation
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Dimension:1Maths:𝔹 equation(·) — A statement that two sides are equal, depending on another quantityDimensionally verifiedMathematically verifiedFully reduced to base quantities

In words:equals(plusminusplus)divided by.

Open full derivation chain: 5 stepsDerivation
  1. 1
    Open Step 1: Given / definition · The derived Momentum PDE (equation 3.24) as the given definition for reduction: unsteady accele…
    The derived Momentum PDE (equation 3.24) as the given definition for reduction: unsteady acceleration plus convective acceleration equals the Powell–Eyring stress terms plus thermal buoyancy minus the Lorentz body force — five relational blocks.Given / definition
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    About levels

    Why the levels load one at a time

    Only level 0 travels with this article. The complete expansion is several megabytes, so deeper levels are fetched one at a time when you ask for them — and how deep it goes is not known until you get there.

    To go deeper still, open any quantity in the reading as its own concept card — it carries its own derivation and its own depth ladder.

    How many levels remain is not yet known — the depth is discovered one level at a time.

  2. 2
    Open Step 2: Similarity transformation · Substitute the similarity maps into EVERY block [eq. 3.40]: u=U_wf′, v=−√(cν_f/(1−γt))f, and T−…
    Substitute the similarity maps into EVERY block [eq. 3.40]: u=U_wf′, v=−√(cν_f/(1−γt))f, and T−T∞=(T_r−T∞)θ with T_r−T∞=ΔT₀x/(1−γt)².Similarity transformation Substitution · η
    Open operand record: Substitution η
    Substitution:L0L2
    Open term-change ledger: 5 records
    Open record 1: ∂u/∂t → γ(cx/(1−γt)²)(f′+f″(η/2))
    Consumes:L0L1Produces:L0L1The similarity transform inserts u = U_w f′(η) into the local acceleration ∂u/∂t; carrying out the time derivative (detailed in the Substituted Unsteady Acceleration node) turns the block into the growth scale cx/(1−γt)² times the transient velocity group f′+f″(η/2).γ(cx/(1−γt)²)(f′+f″(η/2))
    Open record 2: u·∇u → (c²x/(1−γt)²)(f′²−ff″)
    Consumes:L0L1Produces:L0L1The similarity transform inserts u = U_w f′(η) and v = −√(cν_f/(1−γt))f(η) into the convective terms u∂u/∂x and v∂u/∂y, turning the block into the convective scale c²x/(1−γt)² times f′²−ff″ (detailed per-term in the Substituted Convective Acceleration node).(c²x/(1−γt)²)(f′²−ff″)
    Open record 3: τ → τ_PE(η)
    Consumes:L0L2Produces:L0L1The similarity transform inserts the shear-rate and curvature maps ∂u/∂y = U_w(c/(ν_f(1−γt)))^{1/2}f″(η), ∂²u/∂y² = U_w(c/(ν_f(1−γt)))f‴(η) into the Powell–Eyring diffusion and retardation terms (detailed step-by-step in the Substituted Powell–Eyring Stress Terms node), exposing their dimensional coefficients explicitly. Consumed at DEPTH 2 so the second-order velocity gradients (∂²u/∂y², (∂u/∂y)²) that the maps act on are visible, not the opaque τ symbol.τ_PE(η)
    Open record 4: ρgβ → gβ_hnf(Tr−T∞)θcosα
    Consumes:L0L1Produces:L0L1The similarity transform inserts T−T∞=(T_r−T∞)θ with T_r−T∞=ΔT₀x/(1−γt)², preserving a temperature scale distinct from the velocity scale.gβ_hnf(Tr−T∞)θcosα
    Open record 5: J×B → (σ_hnf B²/ρ_hnf)U_w f′
    Consumes:L0L1Produces:L0L1The similarity transform inserts u = U_w f′(η) into the Lorentz body force, turning it into the magnetic coefficient times U_w f′ (detailed in the Substituted Lorentz Braking node); B(t) remains time-dependent at this stage.(σ_hnf B²/ρ_hnf)U_w f′
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    About levels

    Why the levels load one at a time

    Only level 0 travels with this article. The complete expansion is several megabytes, so deeper levels are fetched one at a time when you ask for them — and how deep it goes is not known until you get there.

    To go deeper still, open any quantity in the reading as its own concept card — it carries its own derivation and its own depth ladder.

    How many levels remain is not yet known — the depth is discovered one level at a time.

  3. 3
    Open Step 3: Rearrange terms · Move every term to one side and set the equation to zero [eq. 3.42], the standard homogeneous f…
    Move every term to one side and set the equation to zero [eq. 3.42], the standard homogeneous form: subtracting the stress and buoyancy blocks and adding the Lorentz block collects the whole balance against zero — the SAME five blocks, now equated to nought.Rearrange terms
    Level 0 · more available
    Brief — named quantities
    About levels

    Why the levels load one at a time

    Only level 0 travels with this article. The complete expansion is several megabytes, so deeper levels are fetched one at a time when you ask for them — and how deep it goes is not known until you get there.

    To go deeper still, open any quantity in the reading as its own concept card — it carries its own derivation and its own depth ladder.

    How many levels remain is not yet known — the depth is discovered one level at a time.

  4. 4
    Open Step 4: Non-dimensionalisation · Divide by c²x/(1−γt)² and identify each raw coefficient: γ/c=A; (ν_hnf+1/(ρ_hnfβς))/ν_f=(φ_μ+ε)…
    Divide by c²x/(1−γt)² and identify each raw coefficient: γ/c=A; (ν_hnf+1/(ρ_hnfβς))/ν_f=(φ_μ+ε)/φ_ρ; retardation=εδ/φ_ρ; gβ_hnfΔT₀/c²=(φ_{ρβ}/φ_ρ)λ with λ=Gr_x/Re_x²=gβ_fΔT₀/c²; and magnetic damping=(φ_σ/φ_ρ)M. Every explicit x and t cancels.Non-dimensionalisation Divisor · c²x/(1−γt)²
    Open operand record: Divisor c²x/(1−γt)²
    Divisor:L0L1
    Open term-change ledger: 5 records
    Open record 1: γ(cx/(1−γt)²)(f′+f″(η/2)) → A(f′+(η/2)f″)
    Consumes:L0L1Produces:L0L1Dividing the unsteady block by the convective scale c²x/(1−γt)² leaves the raw ratio γ/c, identified BY DEFINITION as the unsteadiness parameter A — giving the unsteadiness term A(f′+(η/2)f″).A(f′+(η/2)f″)
    Open record 2: (c²x/(1−γt)²)(f′²−ff″) → f′²−ff″
    Consumes:L0L1Produces:L0L1Dividing the convective block by the same convective scale c²x/(1−γt)² cancels its common dimensional factor exactly — that cancellation IS the self-similarity — leaving the dimensionless inertia f′²−ff″.f′²−ff″
    Open record 3: τ_PE(η) → ((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴
    Consumes:L0L1Produces:L0L1Dividing the Powell–Eyring block by the convective scale exposes the raw diffusion and retardation coefficients, identified BY DEFINITION as (φ_μ+ε)/φ_ρ and εδ/φ_ρ (the verified grouping carrying 1/φ_ρ onto BOTH stress groups).((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴
    Open record 4: gβ_hnf(Tr−T∞)θcosα → (φ_β/φ_ρ) λ θ cos α
    Consumes:L0L1Produces:L0L1Using T_r−T∞=ΔT₀x/(1−γt)², division leaves gβ_hnfΔT₀/c². The explicit Gr_x and Re_x formulas identify this as (φ_{ρβ}/φ_ρ)λ with λ=gβ_fΔT₀/c².(φ_β/φ_ρ) λ θ cos α
    Open record 5: (σ_hnf B²/ρ_hnf)U_w f′ → (φ_σ/φ_ρ) M f′
    Consumes:L0L1Produces:L0L1Dividing the Lorentz block by the convective scale and inserting B(t)² = B₀²/(1−γt) cancels the residual (1−γt) exactly, identifying σ_hnf B₀²/(ρ_hnf c) BY DEFINITION as (φ_σ/φ_ρ)M.(φ_σ/φ_ρ) M f′
    Level 0 · more available
    Brief — named quantities
    About levels

    Why the levels load one at a time

    Only level 0 travels with this article. The complete expansion is several megabytes, so deeper levels are fetched one at a time when you ask for them — and how deep it goes is not known until you get there.

    To go deeper still, open any quantity in the reading as its own concept card — it carries its own derivation and its own depth ladder.

    How many levels remain is not yet known — the depth is discovered one level at a time.

  5. Open Step 5: Separation of variables · Isolate the highest derivative: factor f‴ out of the Powell–Eyring stress and divide the balanc…
    Isolate the highest derivative: factor f‴ out of the Powell–Eyring stress and divide the balance by its coefficient, giving the explicit normal form f‴ = … that the numerical solver marches.Separation of variables Divisor · (φ_μ+ε)/φ_ρ−(εδ/φ_ρ)f″²
    Open term-change ledger: 1 records
    Open record 1: ((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴ → f‴ + (φ_μ+ε)/φ_ρ−(εδ/φ_ρ)f″²
    Consumes:L0Produces:L0L0The Powell–Eyring stress ((φ_μ+ε)/φ_ρ)f‴ − (εδ/φ_ρ)f″²f‴ = f‴·[(φ_μ+ε)/φ_ρ − (εδ/φ_ρ)f″²] separates into the common factor f‴ (isolated on the left as the highest derivative) and the effective-diffusion denominator K = (φ_μ+ε)/φ_ρ − (εδ/φ_ρ)f″² that the balance is divided by.
    Level 0 · more available
    Brief — named quantities
    About levels

    Why the levels load one at a time

    Only level 0 travels with this article. The complete expansion is several megabytes, so deeper levels are fetched one at a time when you ask for them — and how deep it goes is not known until you get there.

    To go deeper still, open any quantity in the reading as its own concept card — it carries its own derivation and its own depth ladder.

    How many levels remain is not yet known — the depth is discovered one level at a time.

Equation Workspace

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Domain Analysis

Unit status, dimensional trails, per-step checks, and custom unit expressions.

Step-by-step unit check (5 steps)
  1. Step 1 given
    Expand checked this step.
  2. Step 2 similarity_transform
    Expand checked this step.
  3. Step 3 rearrange
    Expand checked this step.
  4. Step 4 non_dimensionalise
    Expand checked this step.
  5. Step 5 separation_of_variables
    Expand checked this step.
Try your own — the unit calculator

Ever wondered whether a formula “adds up”? This tool answers one plain question: do the units match? Pick a couple of quantities (distance, time, a speed…), join them with ×, ÷, +, or =, and press Run. It works out the resulting unit — for example distance ÷ time gives a speed, LT⁻¹ (metres per second) — and flags anything that can’t be right, like adding a length to a time.

Reads as: f‴

“f‴” is analysed by expanding its full defining equation (its children and their relations), not as a bare symbol. Use Check this concept’s units above for the same result.

Mathematical Analysis

Object type, lawful operations, conditions, comparisons, and derivation-step checks.

Check one step at a time (5 steps)
Step 1 — given
Step 2 — similarity transform
Step 3 — rearrange
Step 4 — non dimensionalise
Step 5 — separation of variables
Equation

Essence

S1The SAME balance after substitution: substituted unsteady plus substituted convective acceleration equals substituted Powell–Eyring stress plus substituted buoyancy minus substituted Lorentz braking.

  • S1.1essenceThe sum of γ(cx/(1−γt)²)(f′+f″(η/2)) and (c²x/(1−γt)²)(f′²−ff″). [1.1]
  • S1.2essenceThe sum of τ_PE(η) and gβ_hnf(Tr−T∞)θcosα. [1.1]
  • S1.3essence(τ_PE(η) + gβ_hnf(Tr−T∞)θcosα) reduced by (σ_hnf B²/ρ_hnf)U_w f′. [1.1]
  • S1.4essence(γ(cx/(1−γt)²)(f′+f″(η/2)) + (c²x/(1−γt)²)(f′²−ff″)) set equal to ((τ_PE(η) + gβ_hnf(Tr−T∞)θcosα) − (σ_hnf B²/ρ_hnf)U_w f′). [1.1]

S2The substituted balance in homogeneous form: substituted unsteady plus substituted convective acceleration minus substituted Powell–Eyring stress minus substituted buoyancy plus substituted Lorentz braking equals zero.

  • S2.1essenceThe sum of γ(cx/(1−γt)²)(f′+f″(η/2)) and (c²x/(1−γt)²)(f′²−ff″). [1.1]
  • S2.2essence(γ(cx/(1−γt)²)(f′+f″(η/2)) + (c²x/(1−γt)²)(f′²−ff″)) reduced by τ_PE(η). [1.1]
  • S2.3essence((γ(cx/(1−γt)²)(f′+f″(η/2)) + (c²x/(1−γt)²)(f′²−ff″)) − τ_PE(η)) reduced by gβ_hnf(Tr−T∞)θcosα. [1.1]
  • S2.4essenceThe sum of (((γ(cx/(1−γt)²)(f′+f″(η/2)) + (c²x/(1−γt)²)(f′²−ff″)) − τ_PE(η)) − gβ_hnf(Tr−T∞)θcosα) and (σ_hnf B²/ρ_hnf)U_w f′. [1.1]
  • S2.5essence((((γ(cx/(1−γt)²)(f′+f″(η/2)) + (c²x/(1−γt)²)(f′²−ff″)) − τ_PE(η)) − gβ_hnf(Tr−T∞)θcosα) + (σ_hnf B²/ρ_hnf)U_w f′) set equal to 0. [1.1]

S3Isolate the highest derivative: factor f‴ out of the Powell–Eyring stress and divide the balance by its coefficient, giving the explicit normal form f‴ = … that the numerical solver marches.

  • S3.1essenceThe sum of A(f′+(η/2)f″) and f′²−ff″.
  • S3.2essence(A(f′+(η/2)f″) + f′²−ff″) reduced by (φ_β/φ_ρ) λ θ cos α.
  • S3.3essenceThe sum of ((A(f′+(η/2)f″) + f′²−ff″) − (φ_β/φ_ρ) λ θ cos α) and (φ_σ/φ_ρ) M f′.
  • S3.4essence(((A(f′+(η/2)f″) + f′²−ff″) − (φ_β/φ_ρ) λ θ cos α) + (φ_σ/φ_ρ) M f′) divided by (φ_μ+ε)/φ_ρ−(εδ/φ_ρ)f″².
  • S3.5essencef‴ set equal to ((((A(f′+(η/2)f″) + f′²−ff″) − (φ_β/φ_ρ) λ θ cos α) + (φ_σ/φ_ρ) M f′) / (φ_μ+ε)/φ_ρ−(εδ/φ_ρ)f″²).

Dimension I

Definition

What it is — and what it is not

What it is
  • S4The similarity-reduced streamwise momentum equation.
  • S5The momentum balance: unsteady plus convective acceleration equals Powell–Eyring stress plus buoyancy minus the Lorentz force.
    • S5.1essenceThe sum of ∂u/∂t and u·∇u. [1.1]
    • S5.2essenceThe sum of τ and ρgβ. [1.1]
    • S5.3essence(τ + ρgβ) reduced by J×B. [1.1]
    • S5.4essence(∂u/∂t + u·∇u) set equal to ((τ + ρgβ) − J×B). [1.1]
Wisdoms
  • S6The reduced momentum ODE in named dimensionless parameters: the unsteadiness term plus the dimensionless convective inertia minus the Powell–Eyring diffusion/retardation group minus buoyancy plus magnetic damping equals zero.
    • S6.1essenceThe sum of A(f′+(η/2)f″) and f′²−ff″. [1.1]
    • S6.2essence(A(f′+(η/2)f″) + f′²−ff″) reduced by ((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴. [1.1]
    • S6.3essence((A(f′+(η/2)f″) + f′²−ff″) − ((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴) reduced by (φ_β/φ_ρ) λ θ cos α. [1.1]
    • S6.4essenceThe sum of (((A(f′+(η/2)f″) + f′²−ff″) − ((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴) − (φ_β/φ_ρ) λ θ cos α) and (φ_σ/φ_ρ) M f′. [1.1]
    • S6.5essence((((A(f′+(η/2)f″) + f′²−ff″) − ((φ_μ + ε)/φ_ρ) f‴ − ((εδ/φ_ρ))f″²f‴) − (φ_β/φ_ρ) λ θ cos α) + (φ_σ/φ_ρ) M f′) set equal to 0. [1.1]
Attributes
  • S7Balances the Powell–Eyring diffusion and nonlinear retardation against convective inertia, unsteadiness, magnetic damping, and buoyancy; the equation is composed on demand. [1.1]

Dimension II

In practice

How to deal with it

Consequences
  • S8The buoyancy term carries θ, coupling it to the energy ODE.

Part of synthesis

Sub-topics 12 / 22

Prerequisite

Related

References

  1. Problem 1 draft synthesis; Aziz et al. (2021) is partial context only, DOI 10.1007/s10973-020-10210-2

    • 1.1

      Independent reduction required — No single cited article establishes this third-order unsteady inclined MHD ODE. Its coefficients and signs are justified term by term in the seeded derivation.

      AmbiguityTenuous

      The cited Aziz article supports only a partial, different configuration; the complete equation is an audited synthesis.

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